I had both an M and an L. The grips on the Steyrs are absolutely fantastic, and the slide serrations are the best. The M9A1's trigger is pretty good but you could buy an inexpensive upgrade that smoothed it out significantly.
The L9A1 model has a better trigger than even that right out of the box. I won't say it's a PPQ trigger, but it's close.
The L also has marginally better accuracy and reduced muzzle flip than the M, but I also had problems with mags not dropping free, failures to return to battery and the spent brass being launched consistently at my head. At the end of the day I found the M to be more reliable, and had a better size : performance ratio. You can also actually find holsters for it.
I've got a pretty in depth thread
here.
All that said, if the Steyr L came out in .357 Sig I would be first in line to buy one.
Also, I contacted Steyr in Austria regarding the lack of available trap sights here and they mailed me a set right away! That was pretty cool.