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.
Oh ya and part of the power savings come from the CPUs being able to get into low-power states easier but they also get into higher-power states and a later gen i5 will have a better ability to "boost" to a higher frequency when workloads demand... so you get a better low and top-end in the later gen i5s - that and the on-die GPUs are a lot better in the most recent i5s (still not great for enthusiast gamers, but good for non-hardcore gamers!)