Of course there were many motorcraft used in the war, but most of them fell into two broad categories. PTs and many MTBs and MGBs such as the Vosper type were 70-80 feet long. S-boats and the Fairmile MTBs were a bit over 100' and 100 tons displacement. There were also smaller types like the MAS or Japanese boats with just a couple of torpedos and machine guns. Naturally the smaller boats tended to be less heavily armed, but the American PTs made an important change in mid-war, replacing torpedos in tubes with Mk13s, originally aircraft torpedos, which only needed light racks. This freed up a lot of weight for heavier guns, up to 40mm, plus things like rocket launchers. This type PT had an armament comparable or even superior to an S-boat. The Fairmile Type D MTB could carry a heavy gun battery including Molins automatic 6pdrs in addition to torpedos, combining the roles of MTB and MGB. The most heavily armed "Dog boats" outgunned the S-boats, with one caveat, I believe the 6pdrs were low-angle only. Also the Fairmiles' torpedos were 18" while the Germans had 21". The S-boats were the only I am aware of having much armor, particularly around the bridge, the so-called panzerkulotte. The S-boats' big advantages were in sustained speed, range, and seakeeping, which at times can be more important than the sexier things like guns. So the choice to some extent depends on where you're operating and exactly what you're doing.