I'd say stick with the third one. You can probably get all the expansions for a lot cheaper the coming months/years now that Cashcow number 4 has been released.
Multitasking Sims and the new Build options are really, really appreciated, but I suspect that the Sims 3 will offer a more "complete" package, especially with its 5000 expansion packs. However, the Sims 3 with its expansions is about 80-85 GB
So you better have a fast internet to dl all dat shit