Grasshopper - I’d say without the strong foundation provided by a Carrier (with two aircraft) your subs become vulnerable, unless they are purchased and tucked away in the Baltic. The Carrier is almost always a lock-down purchase (in the absence of poor dice or poor early decisions on British fleet-killing).
The two transports provides an instant credible sealion threat, and also a credible threat on Leningrad.
A single transport and a sub is a nice option as well, but Britain gains more purchasing flexibility.
The problem with subs is that they are not foundational for your fleet - the 1 on defense is rough. They need to prowl, and hit convoys in places where counterattacks aren’t possible, unless you’re positioned for (and desire) a trade war as a stalling tactic.
So personally (admittedly in Alpha 2) I like them as a turn-by-turn purchase for a strong defensive German fleet - providing cheap fodder and/or cheap offense with the added bonus of convoy raiding.
If I were buying all light ships with Germany on G1, I’d go 3 Destroyers and a Sub… but I’d never do it in the first place against my usual opponent… the threat of sealion is needed to keep Britain honest.
BTW, if anyone here likes good old fashioned fantastic heavy music, try Faith No More’s Album of the Year. It’s great - can’t believe it took me 15 years to discover it. Listening to it right now while I think about my first German turn vs. Gargantua… :)