The last couple of years, there are just 2 versions around. Primo, which is the most recent, and iGO8.3.5.
I can't say iGO8 is outdated, it isn't really, but it is in a way. It's a bit simpler than Primo and skins offer more possibilities.
Primo is newer, has more features, looks nicer, and can be "extended" as well using skins. Only issue here is that Primo does come in some varieties (I mean builds), of which 2.4 (in fact 2.1) is the most recent. If I were you, when choosing Primo as my gps, I would stick to Primo 2.0. P2.4 is not fully developed yet and offers no real differences above 2.0. Yet it must be different because skins are not interchangeable.
Skins do make things easier. Some features that need skins to be activated in iGO are activated in naked Primo. Well; it's a matter of "taste". But skinning is a plus compared to non skinnable nav programs.
For the rest, almost everything (maps, POI, ...) is interchangeable between 8.3, 2.0 and 2.4, restriction for voices (not all voices are interchangeable) and compiled speedcams (codes for redlight cams, speed cams, etc may vary). Not compiled cams are in .txt format and can be compiled in 8.3 or 2.0/2.4.
A list of versions :
http://www.navitotal.com/showthread.php?913-List-of-iGO-8-Amigo-and-Primo-%28Windows-CE-Based%29-releases