Should I get a 50cc or a 125cc bike?

I'm 17 and need some transport on a budget - I'll be commuting in and around towns, for about 60-80 miles per week. I'm aware of the speed differences between the 50cc and 125cc, but will it be a big factor if I avoid motorways?

Also, does anyone have any advice on models? I obviously want to avoid cheap Chinese copies, but it's hard to find reviews for 50cc bikes.

And then the Honda CB125 seemed good in terms of the 125ccs… Thoughts?

Let's see, not knowing your country or state or height or weight or preferred style of motorcycle or level of experience, I suggest you get a used Kawasaki Vulcan 500.
Check Craigslist for your area.

Buy a 125.

Being stuck at 31mph everywhere you go will be incredibly frustrating for you. 16 year olds have no choice - you do. Any 125cc Honda, Kawasaki, Suzuki or Yamaha will be fine. Just find a model you like, at a price you can afford, get your licence and C.B.T. Course sorted out, and get on the road.