first why do you want to sell your VS2 ? In every case this is a downgrade from what you have.
That said, both will perform about the same, and like you said, the xtra only add the foregrip and the expansion chamber, which help to reduce liquid Co2 from reaching the internals. The sonix could be a nice choice, it got the foregrip, but not the expansion chamber.
Now let's talk about upgrades : IMHO there's no point to upgrade thoses markers that much. This get really expensive fast, and for that investment, you're just better with a more expensive marker.
In your situation, check at the new electra '09 : it got the new SE technology which means it can run Co2 at much lower pressure than regular spyders, no really much more than the VS2 (300psi vs 200Psi for the VS2). This means you'll find as much smoothness than the VS2, with the possibility to run Co2 as well.
Think 'bout that
