"preferred" is not really quantifiable in this context. Can you expand on that?
As I've expanded on before in several posts here, here, here, here, here, and here (not an exhaustive list), service as an infantryman in any capacity, in any theater, was extremely dangerous. In this case, I'm assuming here that you mean as a combat infantryman; "infantryman" in this case can mean any man assigned to the Infantry branch, which includes riflemen and heavy weapons men, along with cooks, clerks, buglers, armorer-artificers, and the like, but casualties were heavily concentrated in the former two positions. 93 percent of casualties in infantry divisions were incurred among men serving in the Infantry branch, the vast majority of whom were in the division’s three infantry regiments. In armored divisions, 62 percent of casualties were in the Infantry branch. Armored infantry units were often ridden into the ground due to their high mobility, and suffered from high rates of combat exhaustion. Even with the reduction in the tank-infantry ratio in armored divisions in 1943 (from 2:1 to 3:2), armored division commanders were constantly clamoring for more infantry. 85 percent of casualties in airborne divisions occurred in the Infantry.
No U.S. divisions, as well as none of the myriad of separate infantry and cavalry regiments, engaged in active combat in both the European and Pacific theaters; the 86th and 97th Infantry Divisions fought in Europe for about a month each before being sent to the Pacific in the summer of 1945 prior to the surrender of Japan, but they saw no combat there. The forces which were to be the spearhead of both proposed invasions of the Japanese mainland were those which were already present in the Pacific. I covered the distribution of the forces of the Army in the various theaters of World War II near its end here.
I've highlighted the infantry divisions which served in the Pacific theater.
| Infantry Division | Total Battle Casualties | KIA | WIA | POW | MIA |
|---|---|---|---|---|---|
| 3rd | 25,977 | 4,922 | 18,766 | 1,735 | 554 |
| 9th | 23,277 | 3,856 | 17,416 | 1,648 | 357 |
| 4th | 22,660 | 4,097 | 17,371 | 731 | 461 |
| 45th | 20,993 | 3,547 | 14,441 | 2,527 | 478 |
| 1st | 20,659 | 3,616 | 15,208 | 1,336 | 499 |
| 29th | 20,620 | 3,887 | 15,541 | 845 | 347 |
| 36th | 19,466 | 3,131 | 13,191 | 2,650 | 494 |
| 90th | 19,200 | 3,342 | 14,386 | 1,185 | 287 |
| 30th | 18,446 | 3,003 | 13,376 | 1,164 | 903 |
| 80th | 17,087 | 3,035 | 12,484 | 1,077 | 488 |
| 2nd | 16,795 | 3,031 | 12,785 | 786 | 193 |
| 28th | 16,762 | 2,316 | 9,609 | 3,953 | 884 |
| 34th | 16,401 | 2,866 | 11,545 | 1,368 | 622 |
| 83rd | 15,910 | 3,161 | 11,807 | 663 | 279 |
| 35th | 15,822 | 2,485 | 11,526 | 1,471 | 340 |
| 79th | 15,203 | 2,476 | 10,971 | 1,186 | 570 |
| 8th | 13,986 | 2,532 | 10,057 | 668 | 729 |
| 88th | 13,111 | 2,298 | 9,225 | 647 | 941 |
| 5th | 12,818 | 2,298 | 9,549 | 683 | 288 |
| 26th | 10,701 | 1,850 | 7,886 | 806 | 159 |
| 7th | 9,212 | 1,948 | 7,258 | 2 | 4 |
| 96th | 8,812 | 1,563 | 7,181 | 5 | 63 |
| 85th | 8,774 | 1,561 | 6,314 | 497 | 402 |
| 91st | 8,744 | 1,400 | 6,748 | 334 | 262 |
| 106th | 8,627 | 417 | 1,278 | 6,697 | 235 |
| 78th | 8,146 | 1,427 | 6,103 | 385 | 231 |
| 77th | 7,461 | 1,449 | 5,935 | 1 | 76 |
| 32nd | 7,268 | 1,613 | 5,627 | 1 | 27 |
| 84th | 7,260 | 1,284 | 5,098 | 749 | 129 |
| 24th | 7,012 | 1,374 | 5,621 | 6 | 11 |
| 95th | 6,591 | 1,205 | 4,945 | 380 | 61 |
| 99th | 6,553 | 993 | 4,177 | 1,136 | 247 |
| 27th | 6,533 | 1,512 | 4,980 | 1 | 40 |
| 94th | 6,533 | 1,009 | 4,789 | 619 | 116 |
| 87th | 6,034 | 1,154 | 4,342 | 429 | 109 |
| 43rd | 6,026 | 1,128 | 4,887 | 2 | 9 |
| 37th | 5,960 | 1,094 | 4,861 | 1 | 4 |
| 44th | 5,655 | 1,038 | 4,209 | 308 | 100 |
| 25th | 5,432 | 1,235 | 4,190 | 2 | 5 |
| 100th | 5,038 | 883 | 3,539 | 491 | 125 |
| 104th | 4,961 | 971 | 3,657 | 237 | 96 |
| 102nd | 4,922 | 932 | 3,668 | 137 | 185 |
| 103rd | 4,558 | 720 | 3,329 | 421 | 88 |
| 63rd | 4,504 | 861 | 3,326 | 219 | 98 |
| 75th | 4,324 | 817 | 3,314 | 116 | 77 |
| 41st | 4,260 | 743 | 3,504 | 13 | |
| Americal | 4,050 | 981 | 3,052 | 1 | 16 |
| 42nd | 3,971 | 553 | 2,212 | 1,175 | 31 |
| 70th | 3,919 | 755 | 2,713 | 397 | 54 |
| 38th | 3,464 | 645 | 2,814 | 5 | |
| 40th | 3,025 | 614 | 2,407 | 1 | 3 |
| 92nd | 2,997 | 548 | 2,187 | 56 | 206 |
| 33rd | 2,426 | 396 | 2,024 | 1 | 5 |
| 76th | 2,395 | 433 | 1,811 | 141 | 10 |
| 6th | 2,370 | 410 | 1,957 | 3 | |
| 81st | 2,314 | 366 | 1,942 | 6 | |
| 31st | 1,733 | 340 | 1,392 | 1 | |
| 69th | 1,506 | 341 | 1,146 | 10 | 9 |
| 66th | 1,452 | 795 | 636 | 21 | |
| 65th | 1,230 | 233 | 927 | 67 | 3 |
| 71st | 1,114 | 243 | 843 | 19 | 9 |
| 89th | 1,029 | 292 | 692 | 40 | 5 |
| 97th | 979 | 188 | 721 | 61 | 9 |
| 86th | 785 | 136 | 615 | 19 | 12 |
| 93rd | 133 | 12 | 121 |
In his 1949 book The American Soldier: Combat and its Aftermath, Volume II, Samuel Stouffer, a pioneer of American sociology, measured the attitudes of a sample of American soldiers towards German and Japanese soldiers. It was correlated with their later combat performance in three categories (below average, average, above average, respectively). The soldiers of the sample showed a definite animosity towards the Japanese, but there nevertheless seemed to be a weak correlation between eagerness to kill the enemy and later combat performance, regardless of nationality.
Study S-60 surveyed 3,900 men of the 70th Infantry Division in September 1943, and then again in April 1944, obtaining data on things such as age, level of education, marital status, Army General Classification Test score, and Mechanical Aptitude Test score, along with the survey questions. It was difficult to follow up on the same men once they had participated in combat as the 70th Infantry Division was one of nearly two dozen divisions which gave up large numbers of its Infantry privates for use as replacements in mid-1944. Only 393 men for whom both personnel and questionnaire data were available could be evaluated again in May 1945 when the combat performance survey was conducted.
| Survey question | "How do you think you would feel about killing a Japanese soldier?" | "How do you think you would feel about killing a German soldier?" |
|---|---|---|
| "I would really like to kill a [German/Japanese] soldier" | 38-44-48 | 5-6-9 |
| "I would feel that it was just part of the job without liking or disliking it" | 35-32-34 | 45-52-55 |
| Some other idea or no answer | 7-2-0 | 4-2-3 |
| "I would feel that it was part of the job, but would still feel bad about killing a man even if he was a [German/Japanese] soldier" | 16-18-17 | 41-34-32 |
| "I would feel I should not kill anyone, even a [German/Japanese] soldier" | 4-4-1 | 5-6-1 |
Sources:
Previous AskHistorians answers
Stouffer, Samuel A., Arthur A. Lumsdaine, Marion Harper Lumsdaine, Robin M. Williams, Jr., M. Brewster Smith, Irving L. Janis, Shirley A. Star, Leonard S. Cottrell, Jr. The American Soldier: Combat and its Aftermath, Volume II. Princeton: Princeton University Press, 1949.
United States. United States Army. Adjutant General’s Department. Army Battle Casualties and Nonbattle Deaths in World War II, Final Report, 7 December 1941-31 December 1946. Washington: Statistical and Accounting Branch, Office of the Adjutant General, 1953.