There were a number of major distinctions between the early war nightfighters and the two Sau tactics. For one thing, both Saus emerged in 1943 out of the failures of the early war system to deal with the emerging bomber threat. The Luftwaffe had tinkered around with various methods to intercept aircraft at night, initially relying upon searchlights and ground-control interception to attack the bombers before they reached their target. Most of these aircraft were surplus Zerstorer aircraft or converted bombers. Interceptions were difficult, but fortunately the threat in 1940 and the 1941 was not that dire. The most used tactic in these early days was Hellenachtjagd (illuminated night fighting), which involved using any light source to guide in and attack the bombers. Light sources could range from the moon or air-dropped flares, but most often were searchlights. The Luftwaffe toyed with other infrared searchlights on Do-17 to improve matters, but eventually Telefunken company was able to shrink down radar sets that could be carried by twin-engined aircraft. The advent of radar-equipped aircraft allowed for the creation of a much more intergrated air defense network, the Kammhuber Line, named after its commander Josef Kammhuber. Radar-equipped planes would form a sort of zone defense with ground radar and non-radar equipped planes would illuminate bombers with flares.
The Kammhuber Line managed to accrue some notable successes, but in the electronics war the advent of Window and other British developments managed to blindside the Line in 1943. Worse for the Luftwaffe, internecine squabbling among its procurement officers prevented the development of dedicated nightfighters and the nightfighter arm had to make do with older equipment and not enough of it. The Hamburg raid in July, as well as other developments in the RAF bombing campaign such as the bomber stream underscored how inadequate the German defense network was in 1943. Kammhuber's sacking from the nightfighter arm in 1943 led to new ideas and tactics to be tried out, as well as refining the methods pioneered by Kammhuber.
The Saus were two of the ideas that came out of this new development. Major Hans-Joachim Herrmann proposed to Goering the idea of using barely-modified dayfighters to beef up the night defenses. At first glance, it would appear that the Saus were a return to the early days of 1940/41 radar-guided fighting, but there were subtle differences between the two periods. Unlike the earlier nightfighting, the Wilde Sau were explicitly intended to intercept bombers over their target. Prior practice was to have the fighters intercept the aircraft before bombing. This meant the Wilde Sau pilots flew dangerously close to flak barrages and their Hellenachtjagd methods included burning cities. It was a zone defense, but on point. The Zahme Sau likewise benefited from greater German experience with electronic warfare and guiding. The Zahme Sau, which were mostly twin-engined dayfighters, were guided by radio beacons both on the inbound and outbound flights.
The Sau tactics gained some immediate results, but Hermann's idea was more of a reflection of German weaknesses than innovation. Production shortfalls in aircraft and electronics, a fair deal of it self-inflicted by the Luftwaffe, meant that the Luftwaffe could not field enough specialized aircraft in the numbers needed to meet the RAF. Using dayfighters in a nightfighter capacity made some sense, but it also signaled that the Luftwaffe was clearly losing ground in the material battle in the air. The hope was that dayfighters would not need the specialized and expensive equipment for nightfighting, but this proved to be optimistic. IFF remained a key problem for the Saus and single-seat fighters were inadequate for night operations. The reliance upon instrument flying, which was one of Achilles heels of Luftwaffe training, meant that the Wilde Sau groups were more prone to long-term attrition. Although they managed to gain successes, it was at costs that the Germans could not pay over the duration of the conflict. Moreover, the Saus were predicated on the idea that the bombers would get through, another admission that it was losing the air war.
Eventually, the Saus heyday in 1943 was not to last the following year. Developments in conventional nightfighters made up for the reverses in 1943 and managed to seriously damage RAF bombers in 1944. The shift for the invasion as well as scaling back of Allied night bombing operations because of losses led to the disbandment of the Wilde Sau in mid-1944.
Sources
Heaton, Colin D., and Anne-Marie Lewis. Night Fighters: Luftwaffe and RAF Air Combat Over Europe, 1939-1945. Annapolis, Md: Naval Institute Press, 2008.
Scutts, Jerry. German Night Fighter Aces of World War 2. Oxford: Osprey Pub, 1998.