It depends upon how you use Studio 7.......
If you burn to DVD then S8 provides that function without the need to write an AVI, encode to MPEG2 and burn with a seperate application. The MPEG2 encoder in S8 is almost as good as TMPGEnc.
If you want to edit MPEG1's or 2's
If you are a heavy user of HFX it also provides background rendering, which means it will not stutter when previewing complex HFX's
If you have HFX Plus or Pro the S8 upgrade also automatically upgrades HFX 4.5 to 4.6 which has lots of advantages over 4.5; it updates the Pro version to something better than HFX Gold.
These are the main benefits. If they don't apply or are of limited benefit then stay with S7. It is very worthwhile however to upgrade to the latest version 7.15.1, which was only released a few days ago, but has a major improvement in MPEG2 quality. It is said by many experts to be equal to TMPGEnc but much faster. I have not tried it yet as I do not want to update in the middle of a major project.