Are you sure it's the Agility (not Agility 3 or whatever)?
Either way I think everyone will say M4 if not for speed but reliability.
I have an Agility and an Agility 3 and have no problems with either.(Agility 3 is almost twice as fast as the Agility, not that you can really tell in my usage)
However, will probably buy an M4 soon or the OCZ Vertex 4 as it has a new controller.
M4. never had a problem with mine and speeds are amazing.
every 1 will probably say m4 as well
There seems to be a lot of fans of the M4. I really don't know what it gives over the Samsung 830 though.
It's bulletproof. It's still one of the quickest as well. But i take reliability over a tiny bit more speed that i wouldn't notice anyway 100% of the time.