You said battery life is a problem?
Vista chews through battery life, I had a cheap laptop with Vista, and couldn't even get 2 hours battery life. I deleted it and installed xubuntu and got over 3 hours.
A Macbook should give you much better battery life than a Dell Vista
I would also say that the Mac OS is much better than Vista, and that it's worth the extra price. The extra Ram you get with the Dell will all be used running the OS, but that's just my opinion obviously