A lot of it is personnal taste and your room (placement, acoustics). For my own taste, I usually use PLIIx for stereo movies, Dolby Ex for DD5.1 and THX Ultra2 Cinema for the hi-res coming either bitstream or in PCM.
In all honesty, the move from 5.1 to 7.1 is the one that had the least impact in my setup. I'm glad to have 7.1 (Stargate's BlueRay DTS:HR EX:6.1 has some glorious fly-by back-suround effects), but it remains last in my priorities.
To answer your specific question (with a slight change of algo), I would think that a 5.1DD track in 7.1 with Dolby EX and some M2s at the back would sound a bit better (more real) than a plain 5.1. It's all a matter of how much more (both in $$$ and in enjoyment).
