From my limited experience, I'd have to say 35mm is the only way to go. I have tried TLR, 6x6 and 6x9 Folders. the 35mm wins hands down just for sheer speed and ease of pre-focussing (I know you can do that with all ther others too, just find it easier with a nikkor). I tend to use a 50mm Prime or 24mm depending on situation, my best and most consistent results come with a 50mm, especially protests and up close.
As for enlarging, I have taken FP4, HP5 , Neopan 100 and 400 up to 16x20 and grain hasn't taken from the subject,
Neopan 100 in Rodinal Special 1:30 -- non existant grain- sweet.
my next experiment in Street is going to try and see how I get on with 50ASA film -- which given our dull climate is a big step up from my usual 400 asa, which is the standard for the dull Irish climate.
Final comment who said 16x20 is the magic size? I love 12x16 and 35mm reaches this with little strain.