The Intel’s Core i5-4200U and i5-4200M are Haswell processors ( Successors to Ivy Bridge). The i5-4200U is a low power variant with a Thermal Power Dissipation ( TDP) rating of 17 Watts while the i5-4200M is a normal power processor with a TDP rating of 35 Watts.
The i5-4200M has 900 MHz higher clock frequency ( 2.50 GHz Vs 1.60 GHz) which will seemingly make it substantially faster but for the fact that the difference in their turbo clock is not as high ( 3.1 GHz Vs 2.60 GHz). The lower base clock allows the i5-4200U to run at lower power envelope and preserve battery life. At the same time the processor performance does not suffer if the processes demand processing juice for short period of time.
Benchmark scores indicate a performance improvement of 20% in the M series. You will notice this improvement only when you subject the processors perform to their extremes.
The M series has a slightly better integrated graphics performance.
Check out the youtube video for the comparison.