In day-to-day use you probably won't notice that much.
But power savings did increase w/ each gen. So a later-gen i5 might get you a longer battery life - of course this asssumes all else being equal, some vendors will soup up other hardware in the laptop so the excess power savings are put to use (higher ghz frequency, more I/O options on the boards, etc) instead of put to longer battery life.
Probably its a good rule of thumb to always get the latest when you can afford it.