Well, ummm...I love them both, but I'm going to suggest Gen V as it's new and all. Gen IV's good because they're all released, so anyone who now bought Platinum and Hg/Ss would get the best of the lot. However, Black/White are new, awesome (although I personally do find some things irritating, like not being able to contact the Day Care via phone), and believe it or not, cheaper (£25 new for me in March, whereas Diamond was £27, Platinum was £32 and Hg/Ss were £35).
So, yeah - Gen V ftw! (and it doesn't make you a cheapskate, it means you have brains :D )