If you look at lead acid batteries, their capacity rating is often a 20 hour rating, they might also have a 5 hour rating but very few have a 1 hour rating, 6v Trojan T105 Golf Cart batteries have a 250amp hour rating if discharged over the course of 100 hours, at 20 hours it's 225amp hours but at two hours their they will only put out 146 amp hours and about 100 amp hours at 100 minutes! so is that 250 amp hour battery really a 250 amp hour battery???
That info all came from Trojan's own web site.
http://www.trojanbatteryre.com/PDF/d...ata_Sheets.pdf
If you want highway speeds with 48v you are going to pull a lot of amps so you are going to need larger copper wires both between the batteries and in the motor, you are going to produce more heat all around and have more losses with more weight.
Always go with as high of voltage as you can afford, even if the motor is the exact same motor you can often program speed controllers to limit the peek voltage.
Brushed DC motors are cheap and common but they are less efficient then AC motors so with DC you have reduced range, reduced power per pound, more heat (motor cooling for long rides) and of course more parts to wear, brush wear is not a huge deal if your motor is sized right, heat is not a big deal if you don't ride very far and brushed motors are pretty quite, quiter then a chain on a chain driven electric motorcycle, on my shaft drive motorcycle the tires and wind are still the noisy part.
I would like a 3 phase AC motor on my motorcycle just because it would have better low end torque and would run cooler, but the motor I'm looking at would be close to $3,000 for the motor and speed controller, the motor I'm using right now I spent $200 on used.