Here's a picture of my favourite drive system. Bosch CX Performance paired with a 500 Watt Hour battery.
So many choices - and they all must be the same, right? Mostly 250 Watt or 500 Watt...legal limit is 750 Watt...bigger must be better right?
There are a number of other factors to consider.
First and foremost is nominal output vs actual output.
Bosch quotes a nominal output of 250 Watt for its CX Performance line - but it's actual sustained output is about 540 watts.
Brose quotes a nominal output of 250 Watt (and 350 Watt for Speed Pedelecs/28 mph) but really it kicks out 340 Watts.
Suntour quote 450 Watt for its HESC E45 system but really it kicks out 750+ Watts.
Bafang quotes 750 Watt for it's BBS 02 mid drive, but you can plug it into a laptop and go 1,300 Watts.
Conclusion - nominal output is meaningless.
Second: Is this going to affect my range?
Yes, absolutely. The more power you are sucking out of the battery the faster it will deplete, and not in a linear fashion. Higher strain on the battery means higher heat. It will deplete faster than just the increase in wattage suggests.
Plus there's speed. At 28 mph you need twice the power to push through the air as you do at 18 mph - so your effective range halves.
But! There's always a but...
What about efficiency?
Here's a pretty picture of a Brose mid drive. 350 Watts is plenty of oomph to cruise around. Combine it with a mid drive and some excellent software and you end up with the best range package I've seen. Genuine 60 or 70 miles.
So what about that fabulous Bafang! 1.3 KW!! Well that level of output kills your battery in about 20 minutes. Unless you hit a hill, in which case things are going to get very hot. Great power, but seriously curtailed range. It's not a very efficient system...even if you turn the power down...but fabulous for the home build tinkerer! I run one on my dog walking bike and love it.
The Bosch at 540? Super efficient. Gets great range considering the power output. Joyous. Slight whine...
The Bosch powered bike goes twice as far as the Bafang at the same fairly high speed, even with a smaller battery.
Third - All three above are mid drives....what about a hub drive?
Down below there's a picture of the Suntour HESC E45. It says 500 Watts on the side, but it also says 48 Volts by 15 Amps. So 720 Watts then. But actually fully charged it's 54.6 V x 15 A = 819 Watts. But it's a hyper efficient internally geared motor, spinning lord knows how fast, and it can do 25 miles at 25 mph. Which is outstanding.
There's some other great stuff in hubs...but a non-internally geared hub on a hill turns very slow. Slow turns plus hill = high amps = high heat = high battery consumption.
Magnum's Das Kit 500 Watt is another great example of an internally geared rear hub (fast internal spin = efficient).
But internally geared rear hubs can make some noise - slight high pitched whine.
Big ol' Stromers and Specializeds use nice rear hubs that aren't internally geared. Not very efficient.
Also you can't use suspension with a hub.
That being said...it's hard to get decent fenders on rear suspension - though that is changing - so now we move into all sorts of other wrinkles!!
Conclusion: Mid drives rock. Bosch or Brose.
Bosch if you want a sports car
Brose if you want a long range, silent Merc
Bafang if you want to tinker.