I've used both tarp/bivy and UL tent (300winmag's old Contrail, as a matter of fact), and the choice largely depends on the weather forecast; for weekend trips or longer when there's a significant chance of rain or the forecast is unknown, I take the tent. For a short trip in good weather I'l take the tarp/bivy. I should probably add that I use an Equinox ponchotarp rather than a dedicated tarp because I like the multi-use aspect and I can fit underneath a simple lean-to setup.
The caveats are that I hike in the SE US, so most if not all of the time I'm hiking under a fairly thick tree canopy that will deflect heavy rain and winds. My hiking style also keeps me moving until after mosquito feeding time, and the bugs aren't too bad here compared to other places.
I think the final determination comes down to style and budget. If you've got the dough to shell out on a quality tarp/bivy setup and tent, great (I was, thanks to the used gear forum here). Choosing one setup will require a careful analysis of how long you're out (and whether or not you're willing to sack a trip based on a bad forecast), what kind of terrain you'll be in, the bug problems, and your presonal limits of confinement (sleeping in a bivy, tarp or no, can hardly be described as "spacious"!).