[DeepSix]

I don't know how much of this you already know (probably all of it). Porpoise is slightly farther back on the evolutionary chain than Sargo. The second batch of V-class boats (Argonaut, Narwhal and Nautilus) was produced in the late 1920s and led directly to the experiment with Dolphin; that, in turn, led to Cachalot and Cuttlefish, they led to the P-class around 1933, and soon after that the Salmon class was introduced (which included Sargo).

Argonaut was purpose-built as a minelayer; neither Narwhal nor Nautilus was ever able to maintain their 17-knot specified surfaced speed, and submerged they could just barely top 6.

Argonaut, Narwhal and Nautilus - 370-380 feet long, displ. 2,700 tons. 180,000 gallons.
P-class (Porpoise) - 300 feet/1,300 tons. 90,000 gal.
Salmon (Sargo) - 308 feet/1,450 tons. 110,000 gal.

So one's a little longer, one's a little heavier. One could make a case for Sargo since it's got slightly more range and the V-boats had lots o' that. On the other hand, I'm not sure which one dived faster or turned tighter. Bottom line, I don't really think it matters which one you use since it's a stand-in. I'll vote Sargo, but really I'm just looking forward to having a V-boat mod!

[Edit: Just saw your last post. If Sargo is that much done, I'd just stick with that (sorry to the Porpoise voters)]