I've installed a few different one's recently (helping out a mate with a bike shop). from what I've seen, hub centred drive is the way to go, purely for efficiency, front or rear wheel doesn't really matter but front is easier to fit (not that rear is especially difficult though). bigger battery means bigger weight, so decide what you want it for, if it's for a few quick nips to the shop get a smaller battery and charge it more frequently, if it's for epic day long rides, bigger the better. look closely at the specs, if they claim more than about 350w then don't bother, also decide if it needs to be pedal assist or throttle.
cyclotricity have a 250w front wheel conversion which was nice to install and worked well (and didn't look like a conversion to much), the motor and control box are cheap as chips (relatively) but the battery is where the cost is.
you'll see a lot of other kits that are rebranded from their stuff, they also have a 250w/500w rear kit, great fun, but eats the battery.
pannier batteries are fine, just a bit awkward and means you have to have a pannier rack fitted (obviously) all the time, this bothers some people.
any of the kits that rub on the tyre/rim aren't worth it, massively inefficient and very noisy (not to mention the additional wear on tyres and the requirement to have slicks).
there are a few out there (not that I've seen any) that link to the crank, never had any experience with these though.
good luck with the cycling forums, you'll find all the Jacobs you need (no offence intended Jacob).
