350, I rode up an 8 mile 3000' pretty gnarly ascent with a 2wd bike (350w rear 250w front) in less than an hour. Not super fast, not super steep (but about what I would do with my 750w mid-drive). The only problem was the hub motors made the bike feel like they were made of concrete on the descent. My 1000w rear hub (cheap YES system) screams up fairly steep hills. The only problem with rear hubs that I can perceive for ascending is really tortuous, craggy hills, and I'll be trying that out soon (BTW, I'm running a 52V battery, 30 amp controller, so 1.6 kw or so). Problem with bigger hub motor would be more weight for descents. You seem to be doing well with your 350 BionX. If there were a good dealer near me I'd probably have a 500 and use it for everything but some of the more severe off road activities.