The main differences are Studio 8 includes:-
MPEG1 and 2 editing
Background rendering of HFX transitions and effects (smoother preview and faster edit rendering)
DVD authoring and burning
If these are important to you then buy Studio 8. I suspect that Studio 7 will be phased out after a while, so for longer term support it it may be better to buy Studio 8.
Both Studio 7 and 8 are the most capable packages and with HFX can produce extremely professional results.