That's the main thing.
IF the gearing is the same (I have no idea if it is, but it would make sense in a budget model) the
200 will perform A LOT better at any speeds when you press the loud pedal.
I wouldn't worry about spending more on the 163cc model since it's more then likely to outlast
the 125, and consume less fuel at steady cruising speeds.
IF anyone can confirm that the 125 and 200 share the same gearing/CVT setup, it's very likely
that a simple roller swap (going to Dr Pulley sliders) would make the 200 fly.
However, I've ridden the 200.. and it's best suited for cruising at 50-55mph, and wasn't able
to reach the manufacturer stated 108km/h with it (it topped out around 105km/h for me, but
owner of the bike says it will do up to 115km/h on flat ground/no wind).
The 125 probably can't do that.
My Agility 125 would top out at 110km/h (9hp) but the speedo is very inaccurate.