Laptop - i7 or i5?

[Daz]

Does anyone have any experience with the i5 and i7 mobile chip(and sets)s? I'm looking to get a new laptop this year, and while I originally started looking at 13" models (portability is important, I'm off to a few geeky conferences this year and will hopefully do that sort of thing regularly), I'm finding it difficult to justify the inflated price for older generation tech and a screen resolution (9 times out of 10 1280x800, the other time it's 1366x768) that doesnt float my boat. The current crop of ~15"ers should be lighter than my current D820 so that'll have to do

Dell have some good offers on the studio 15 at the moment, along with double quicdo (6%) and a 5% voucher I can use, plus interest free credit, which is a bonus. I cant decide if I should cough up the extra for an i7 though - 8 threads is tempting, but I obviously wont be maxing that too often. I wonder if there's a noticeable affect on the battery life. It seems clever enough with it's power management (as core usage count goes up, clock rate comes down), but I'd be interested in the opinions of anyone who's lived with one, ow who knows about such things

So, should I discount the i7 given that I need this thing to stay awake more than I need it to crunch the **** out of some numbers, or would the difference be negligible?