Altiverb 7 Regular and XL sound exactly the same and come with the same IRs.
There are three differences:
Altiverb 7 Regular supports
- up to stereo input and output
- sampling rates up to 96kHz
- plug-in formats on windows: aax native and vst, on mac: aax native, vst, audio unit, mas and rtas
Altiverb 7 XL supports:
- up to 5.1 surround in and output
- sampling rates up to 384 kHz
- all plug-ins formats Altiverb 7 Regular supports + TDM on Mac OS X
So, in short, if you do not have sessions on 192k and you work in stereo only, Altiverb 7 Regular will do just fine.
If, at a later stage, you would like to upgrade from Regular to XL this is possible. The upgrade cost is just the price difference so you won't loose any money going for Regular first.