I've got a military bivy too...but I got a top zipper sewn on (most shoe repair places can do this kind of stuff...get a ykk zipper).

while a tent is going to be roomier it's also heavier and more cumbersome to put up (especially at night in the wind/rain)

I leave my air mattress (xllite) and my sleeping bag (feathered friends Hummingbird) in the bivy and roll the whole thing up and stuff it at the bottom of my pack.

with the bivy you just throw it out and dive in (zip up) then blow up your mattress while you're inside (extend the mattress on top of yourself...blow it up and then roll on top of it...super easy once you do it a couple of times).