Very little, except the 755m has better memory bandwidth.
http://en.wikipedia.org/wiki/GeForce_700_Series
There appears to be a RANGE of bandwidth given for the 750M with the lower range likely for DDR3 and higher for DDR5. The 755M should be barely different from the 750M with DDR5 memory but if the 750M has DDR3 memory this may cause a bottleneck at times.