SSD performance is trade off. Intel has optimised performance for what matters to 99.9% of desktop users, i.e. random read and writes of small files. OCZ, Kinston etc quote high sequential read write speeds, but they are only of any use if you happen to be writing or reading large files, which is something most people don’t do with small capacity ssd’s.
Personally I look for low latency and high 4k read/ write speeds at queue depth one. The sequential speeds that marketing people quote mean very little, although they do of course sound impressive.
The X25-E uses slc nand as opposed to mlc nand for the M version drives. SLC is a lot more expensive and it has better endurance, hence you can have your cake and eat it with the X25-E. (Albeit a very expensive cake and way over typical specs for anything non enterprise.)
Everything above is correct. However, the speed difference between the X-25M vs the X25E is the speed of the Nand, not the firwmare. SLC Nand programs typically 2x - 3x faster than MLC with some minor benefits for both read and erase speed.
Also, the X-25E has a much larger "spare area", that is extra space not given to the user. This allows more space to shuffle data around which makes random performance even better.
Lastly, the SLC's greater endurance allows it to last much longer.
Of course, all this isn't free. But the X-25E is overkill for a client usage model, like your own laptop. However, it may be appropriate for heavy-duty servers needing high performance in a 24/7 enviroment.