For hilly area's lots will say that a mid drive is the only thing and gives a more natural feel. Frankly that's guff, a hub will handle things just as well and is generally less hassle.
If looking at off the shelf bikes be aware that some (Bosch, Panasonic) are 'locked in' systems. That is if anything goes wrong you are wholly dependant on manufacturer replacements (if they can be got) and they are always way more expensive.
If going the conversion route, plenty of help to be had here.
Personally, I have a front hub conversion that's got me around hilly West Wales for the last 7 years. It does have it's foibles, but then don't we all?