I’m not sure of your context.
Are you thinking of overvolting your current motor, or of buying a complete bike (or package) with either 36 or 48?
If the latter – choosing between the two:
I chose a 36v, because it is lighter, cheaper, and because I can put out 1000 watts with my legs if I need to (or I get pissed off, lol).
48v is better for most people, because it requires less current at the same power (as numerozero said above). Heat is why ebikes fail.
If the former
If you are taking what you have now and converting it – you’ll need a new controller. One feature of the controller is LCV – cutoff at low voltage. This keeps you from destroying your battery from running it empty (although to some degree the battery BMS should help with this). Also – motor speed is directly proportional to voltage. Going from 36 to 48v on a motor that spins at 20mph, will make it want to spin at ~27mph. But if you ask it to do this and it can’t – well something is going to overheat and you will fry your setup.
But, I do have a motor rated for 200rpm at 24volts that I run at 36 volts (300rpm). I can get away with it because I have no hills on my commute and I’m not gonna bog the thing down.