The 125i is very nice without any doubt, but I guess still a "normal" car with motivating output, but no breath taking shove in the back either.
The M135i is no longer a normal car, output is +40% more over the entire rpm range putting it at eyes height of real sportcars. The M135i is the master of the M3 up till 5000 rpm for what engine output concerns. To me, this is a serious reference.
Delta in price is too small to not opt for the finest variant to my opinion. Never before, such a package was available for this cost. When would be in the market now, it would be this one.