Don't fit a front motor if you can avoid it. It will spoil the suspension on the MTB and affect the steering a bit on either of them. Front motors sound louder and have less traction on steep hills. They're OK, but a rear motor is always better.
Oxydrive do a rear cassette motor kit that will allow you to keep your gears, or, if you want 50% more power, you can get the 36v 201 rpm Q128C and run it with the 48v battery and controller to get. The two are perfectly matched to give good torque for hills and good speed (20 mph cruising).
motor wheel
https://bmsbattery.com/ebike-kit/769-q128c-135mm-500w-rear-driving-ebike-hub-motor-wheel-ebike-kit.html?search_query=q128c&results=2#/213-rpm-201/42-voltage-36v
battery
https://bmsbattery.com/ebike-battery/774-48v116ah-case-08-bottle-panasonic-battery-pack-battery.html
or this battery:
https://bmsbattery.com/ebike-battery/680-48v116ah-bottle-09-panasonic-battery-pack-battery.html
Whichever kit you get, you should add a torque arm, which you can get fro Ebay, Cyclezee or here:
https://bmsbattery.com/ebike-parts/450-a-pair-of-ebike-torque-arm-parts.html?search_query=torque+arm&results=1
That BMSB kit has a nice LCD and everything you need, but most of all, it gives you flexibility over power and speed settings to make a very user-friendly kit. The motor is not strictly legal, but it's the same size as most legal ones, so nobody would know if you pealed off the sticky label. You can be sure that exactly that motor is used on some bikes somewhere in Europe that have EN15194 certification. It can be set to 15.5 mph in the settings if you want.
If you were sure that you wanted to stick to 15.5 mph, you could run it with the equivalent 36v batteries, though you'd get less torque too (75%).
Out of those two bikes, the hybrid would be a lot faster and more efficient. I fitted an Oxydrive kit to the previous version, and it was very fast - easy to sustain 20 + mph with minimal effort and easy to pedal without power. MTBs are OK up to about 18 mph, but then you can feel the difference, so choose the MTB if you want to go off tarmac/concrete roads and the hybrid for fast travelling/commuting.
Here's that bike on the Gadget Show. You can see the difference compared with a Volt MTB (restricted):
