Someone is offering me a 400W hub. All my buds have 500W hubs. It's still 36V, like the others. Is there any real difference between 400 and 500 watt?

there is a definite difference. If they are rated accurately, wattage is what really determines how strong the motor is, not voltage.

The one rated for fewer watts will either have less torque (as in, steepest hill it can handle is smaller) or less top speed. Or both.

there is a definite difference. If they are rated accurately, wattage is what really determines how strong the motor is, not voltage.

The one rated for fewer watts will either have less torque (as in, steepest hill it can handle is smaller) or less top speed. Or both.

Well, other factors come into play. Design differences, such as geared (Heinzmann, Ezee) or direct drive (Crystalyte, Wilderness, Golden) can have *huge* impacts on how well these motors perform. Stokemonkey does with 430W what a Crystalyte 5 series does with 1000+, for instance, mind you Stokemonkey isn't applied as a hub motor though.

But in this case, if you're referencing a typical 400w direct drive brushless hub motor, there will be a notable difference in performance between similar models with one operating at 400w and one at 500w.