This is a vague question as Habsburg Spain could be anything from the first decade of 1500 to 1700. A more precise date would be easier to answer. Since I am a scholar of the first half of the 1500s, I'll give you my interpretation.

If you are a real soldier, and by that I mean one who has formal training and maybe a campaign under your belt, it would be more financially profitable to stay in Europe. By the 1530s, there are numerous places that one could find a position as a soldier in Charles's army: the German states, the ethnic minority states in the Iberian peninsula, and even against the Ottoman advance into Eastern Europe. Tracy's "Charles V: Impresario of War" shows that soldiers were getting paid or given valued promissory notes during this time. Why leave when money and advancement are knocking on your proverbial door as a professional soldier? Fine, you want to leave.

Let's assume you are a trained infantry soldier with basic experience. You can fight with a sword, use a pike in various ways, and maybe understand basic fortification construction procedures. You meet a captain (or well financed individual calling himself by that title) in Seville. He needs men to go to the Americas. He is desperate, not because there is a lack of manpower, but there is a lack of men with real knowledge of combat. Many "soldiers" in the Americas are lower middle class folk and some farmers with illusions of grandeur as combat troops. They are clerks with swords. Their knowledge of combat comes from the tales of Reconquista fighting generations past or from fighting indigenous warriors in the Caribbean or coasts of the mainland. Restall's "Seven Myths" highlights how inept some of these men were.

But our man is skilled at fighting. He will be directed to the Casa de Contratación (now the site of the Archive of the Indies) in Sevilla. After proving he is a good Christian and not recently converted from the Islamic or Hebrew faith, he will get an official pass granting him permission to go to the New World. Again, this might be denied if our man's skills as a fighter are so well established that the Crown demands he stay in Europe. As a trained soldier, he will have equipment that he might have purchased or acquired through campaigns. These items mark him as something special. Regular men coming to the Americas might have a sword or access to horses, but few have quality weapons, pikes, crossbows, or an understanding of artillery like our man might have. Because of this, the captain will most likely give out man a position of minor authority. He will also get a larger share of the cut from whatever riches are discovered or taken during their entradas (expedition). If he is lucky, our man will be on a entrada with a group heading to Peru or Santa Marta where the riches are plentiful and the indigenous resistance light compared to what is found In Mexico from 1519-1521. I would look to Varon's "Francisco Pizarro and his Brothers" to see how well connected or well equipped men found riches and brief success in Peru in the 1530s.

There is a window, however. As Kris Lane's brilliant edited translation of Marchuca's "Indian Militia" describes, men who are late to the party face a long, violent struggle of consolidating Habsburg power against Indigenous forces who operate with less devotion to native imperial or religious leaders than to ethnic and cultural preservation.

Again, this is if we limit our soldier to the first half of the 16th century. I am fairly confident that it could describe the second half as well.