In reply to abr1966:
I'm guessing the lack of responses may be because people are a bit mystified by your 'decision'. A tablet will involve a lot of weird compromises depending on how you think you are going to use it (aka your 'usage model'). Is it to be you primary nav tool? Or just for occasional use in addition to a paper map?
If the former (assumed, since otherwise you wouldn't be too bothered about battery life), then a primary consideration is how you're going to carry it - pocket (then it can't be much bigger than a phone)? Map case round your neck (will tend to get banged around a bit with a high risk of cracking the screen). Some sort of dedicated protective (non-transparent) 'tablet-bag'?
I regularly use paper maps or phone for primary nav tools (depending of what I'm doing), but I'm struggling to imaging a situation where a tablet would be better than either. Perhaps I'm too set in my ways...
As for battery life - I don't think there's much difference between cheap tablets (I certainly wouldn't be packing an expensive one up the hills unless I really, really had a good reason), so the answer is likely to be an external powerbank.