Try 'em all. Use what sounds best to you. 
Yes, they all use the same approach: mono correlated (in-phase) content goes to the centre speaker, decorrelated (out-of-phase) content goes to the surround speakers. Since each one was invented by different designers, they'll sound slightly different, but all of them are compatible with Dolby Surround matrix encoding.I know they work differently for normal non matrixed stereo, but for things encoded in matrix will it properly decode them?
No, those are surround virtualizers, which use psychoacoustic processing to give the impression of surround sound when using only 2 speakers.and talking to a friend of mine he says his settup has SRS TruSurround HD and SRS StudioSound settings. Apparently those do matrix stuff too? Would those properly decode dolby surround from laserdiscs?
Easiest way to learn is to listen for the differences with each upmixer.I know the easy thing to do would be to use PLII, and i likely will just use that, but I want to learn!