In my opinion go 10.5, but leave previous versions for 10.4 users. It'll also save you some time focus on one version.
I'm a bit confused as to what is new in v2.0 of Vice. I see the new smaller titlebars and the inspector. Anything else?
I would by the way love to be able to use cmd+1 and cmd+2 for switching the screen to normal and double size, like the Quicktime Player.
I would also like a "favourites" tab, where you can put a list of favourites in a list, maybe with screenshots.