Dennis welcome! Whenever you say prewire, is your room existing or under construction?
Whenever I pre-wired my 25'x14' room for the sub(s) I routed the RG6 coax to four locations ... 1-Left mid sidewall, 2-left front corner, 3-right front corner and 4-right mid sidewall and hindsight being 20-20 I should have pre-wired both rear corners as well, just for good measure.
*If* I were you and it was up to *me*, I'd pre-wire all four corners and both mid sidewalls.
Two 350s will not be better or equal one 500 due to the output level of the 500 and how low it can go but 2 350s will fill your room better than one 350 but then again if I were *you* I'd most certainly opt for the 500. You can always add another at a later point in time because the room is already pre-wired for it.
