Is there a reason you want to geared vs DD or is it just a personal preference thing? I've only owned one geared hub but it was a generic no name hub in a 20" setup and even on 50V I found it had no performance. Haven't had the pleasure of riding a BMC or a Mac motor yet which I seem like way more fun. Are you just planning to go slow and do a lot of climbing? Thermal management becomes an issue in any low RPM high load situation and while geared is more efficient at low speed and they are pretty poor at shedding heat.
Pluses and minuses to each type. Downside to a big motor like a Cromotor (and a few other large DD hubs) is lack of rear gearing choices and it weighs 26lbs, but I've found running a triple front chain ring I could get around a lot of the single rear gear issue. I don't off road though so weight doesn't matter much to me. I also ride a 26" setup with a 9C 2808 and am putting a 9C 2806 into my wife's 20" bicycle.
Greyborg Warp Frame with Greyborg Cromotor aka "Monster Bike
", 24 FET IRFB4115 EB324 Top Speed 42MPH (75V)
Greyborg Hub Motor, ZombieSS 24FET/36FET IRFB4115, Customized full suspension kids MTB. Top Speed 61 MPH (125V)
9C 8x8, 24S2P LiPo, Lyen 12 FET, Diamondback Recoil Comp. Top Speed 42 MPH (100V)Greyborg USA/Canada Dealer, Frame kits + motor in stock now http://www.greyborgusa.com