Pretty silly question, of course the i5 will have better battery life than the i7.
I'm not sure how something more powerful, with more transistors and much beefier GPU would use less power. Barring a device design error.
Most USB battery packs are 5V. Only the Surface 3 (non-Pro) can be charged with that. The SP3 and SP4 requires a 12V battery pack. Same with the SP2 as well.My understanding is that the SP3 and SP4 can both be charged by high quality USB battery packs though, so that's an acceptable alternative to me so long as it works..
Id recommend getting the 8 GB SP2. Not only because its your ideal size (size matters, a lot), but its also a lot cheaper by now, although you may need to buy used. The second candidate would be the 8 GB SP4, but you lose the Power Cover, size preference, and cost advantage.I still prefer the smaller screen, for increased portability.
In all honesty, my SP2 is plenty powerful enough for me, but I'm often hitting the limits of my 4GB RAM limit, so if I'm gonna upgrade, 8GB is mandatory.
Computing power is very important to me, but since this is primarily a mobile platform, battery life is everything. If the i7 gets an hour or more less life off of a charge than the i5 would, then I'd probably rather have the i5.
Just as a heads up, pay absolutely no attention to the time remaining figure under battery. Their algorithms are completely fubared. If you run you system hard to 2 minutes Windows decides that this is how the machine will be used til it is empty and so just tells you 3 hours left. 5 minutes later it'll tell you 10 hours. It really is meaningless.