Take a look at
Sparky's Crash Course for a summary of info.
I would say that
GEBE and
Staton are the two top vendors from a quality standpoint. Both vendors take pride in their products, and only sell high quality, EPA/CARBII approved motors. Honda, Robbins-Suburu, Tanaka, Mitsubishi, and the like. Their kits cost a little more, but, you DO get what you pay for.
Ease of installation is with a friction drive system, first, probably GEBE (Golden Eagle) second, and Staton chain drive third.
Staton makes a friction drive. I use a Staton friction drive - it is of excellent quality, and is very easy to install. However, there are a few caveats regarding friction drives. First - they are less efficient than either chain or belt drives. The drive roller presses into the tire, increasing the rolling friction of the wheel. Second - The tire will also wear somewhat faster than with a chain or belt drive. Not only is the tire rubbing against the ground, it's rubbing against the roller too. Third - they don't work as well when they're wet. You get more slippage on the roller, and in order to account for this, you either need to avoid rapid acceleration (go easy on the throttle) or increase the roller pressure against the tire. It's very easy to do with the Staton. If you want to peddle, there is a noticeable drag unless you disengage the roller. On the Staton this takes about 10 seconds.
GEBE belt drives are also fairly easy to install. They do take more effort to get them mounted than does a friction drive, and, the toothed pulley takes some finagling to get it centered correctly on the wheel. Also there are reports of a higher rate of spoke breakage than with the other two, and the belt is susceptible to dying. Also, if you tend to 'push' the motor more at startup, you tend to see reduced belt and spoke like. Seasoned GEBE riders emphasize avoidance of jackrabbit starts. And, there is a definite upper limit on HP with the GEBE system. Less than 2 HP is recommended. Also, if you want to peddle, there is a noticable drag unless you remove the belt. (granted, this only takes a moment.)
The Staton chain drives take a little more effort to get installed than the GEBE belt drive, but, they are VERY rugged. He uses the same gearbox on his motorized wheelbarrows that can haul more than a thousand pounds, and the gearboxes come with a lifetime guarantee. There's no HP limit either, and you can do the jackrabbit starts as often as you like. A freewheel sprocket on the rear wheel is used, which means that you can peddle at any time with no resistance, as the freewheel will just tick along.
With both the GEBE and Staton systems, you will probably want to make the rear wheel a little beefier - get heavier 12 gage spokes installed. On the friction drive systems, a smooth rear tire is recommended.
Sometime, in the not-too-distant future, I'm looking to move to either a Staton chain drive, or a GEBE.
BTW. There was a recent discussion regarding
staton vs. gebe in this forum.