I'm echoing Soren's advice here, but also want to explain that VD's single player is, in my opinion, better than V's. But both aren't really worth buying for the single player alone unless you're a die hard fan.
V's story is stilted and weird, very hard to get into and very poorly explained. The voice acting on the INT copy is very bad too. VD's story had significantly better characters and is composed much more cleanly as a story. The big difference is content. Verdict Day offers far more single player and multiplayer content, in my opinion, because most of the missions in V last only 30 seconds or so, except for the story missions which are longer.
VD has more subquests and normal missions, more maps, and hardcore mode as well, so even as a purely single player experience there's a lot more content and replay ability. Combine that with the fact that V's online servers are no longer being ran, and VD does have a sort of active group on the PS3, it seems a no-brainer to me.
So in my opinion, if you have to choose between V and VD, it's worth getting VD. But if you have to choose whether to get an 5th Gen AC game at all... then that's a more difficult question.
For single player, $30 on VD is going to get you a few week's worth of content with a steep learning curve, if you include hardcore mode. If you're cool with that, then by all means, buy it.
But if you're looking for multiplayer you have to make friends, you have to spend time learning, and you have to be actively talking with others to get much use out of it at all. There's activity, but most of it is done via friend requests and invites. So if you're looking for an online game, you need to commit to it.
tl;dr: Get VD, not V. Single player is a few week's of content for $30. Multiplayer requires hours and hours of strong commitment, but is very rewarding.