It's slower because it simply just is, initially the gap was even bigger.
But with the 0009 firmware, Crucial has brought the M4's performance pretty close to Sandforce based SSDs.

However, Sandforce based SSDs do have a detrimental impact on performance (again, I stress, only in benchmarks) over incompressible data.
The Crucial M4 works just at its rated speed regardless the type of data being processed.

Finally, although the latest firmwares for SF-2281 controllers has pretty much almost alienated the BSOD issues, there are still more users facing problems with SF-2281 based SSDs than there are on Marvell based SSDs.

If you need to know more, do some more research on Google.

As a final answer to your question, I still suggest getting the Crucial M4 despite being a little bit slower overall.