John
The simple answer is you need a Roland Sound Canvas to play back GS MIDI files. For XG you need a Yamaha sound card to play those back.
GS is a Roland addition to GM MIDI. It allows for banks and a way to access them. XG is also an addition to GM but is for added filter and control to the GM spec. Neither one will play back correctly on a GM device..
GS has unlimited patches via it banks. GM does not recognize XG MIDI files. GM has only 128 possible patches. It is possible to play GS MIDI files on a GM device. It wont sound as good.
GM 2 has 256 patches .
I'd salt that advice bit.
Since I play with foreign midi files all the time, and happen to have an MU100, I can say that 'XG' files pretty much follow the GM/GM2 standard. Both GS and XG extend GM with banks, but you'l find many files with GS or XG sysex data yet are still using the basic GM bank with no banks specified for the program changes.
XG vs GS - XG is generally easy to follow, bank numbers bring up variations on the same instrument, and bank numbers represent a standard variation, i.e key-spread, stereo, etc. Standard behavior for XG samplers is to default to basic bank 0 for any bank not supported in a particular model, giving the proper instrument if a variation is not supported. With GS, the banks don't quite follow a scheme - you really need to know the program list (which you can google up).
It's not all that hard to repatch a GS-based file to a GM/XG one within Sonar, I do it all the time. Of course, no roland sysex data will work, but it's very rare to find one that has anything more than a simple GS reset, which you can safely ignore or remove. Sysex data is just quite rare...because it's model specific.
I'm not sure, but I think the TTS-1 is GS compatible to some degree... It's got the high numbered banks of GM2 - and you can load a midi using that to get the instrument lists, before you repatch to a vst or an different external rompler.
For an XG unit, you can pick them up on eBay for $80-$100... I just sold my MU80 for an MU100 there...it's worth it if you play with midi files.
As for differences in sound - excepting special, advanced controls such as ADSR envelopes, differences are due to the samples themselves. Nothing you can do about that. Then again, no two musician performers ever sound exactly alike either.