I just built a system recently with two M4s 128gbs. I guess the advantage of a 256 is you get more storage for a little under the cost of two drives. Though if you were to go for 2 128gb drives and raid them you'd see better performance.
Only in sequential read/writes... depending on your raid controller, you may even be hampering small I/O speeds in a raid-0 configuration.
For the tiny gains in real world use in sequential rear/writes and potential downfall depending on raid controller... it's not recommended to raid SSDs.
The problem I encountered before I had my M4s (My Vertex2e) was that using it at as a boot drive it needed to be quick, I needed to compensate a lot by leaving at least 20% as a minimum free to retain the speed. On top of that if I installed the likes of Steam onto that drive all my steam games had to be on that drive to, so I ended up having storage problems with a small drive.
A quick google would have shown you how to create folder links in windows 7 - it's very easy. There's even a tool out now that does it for you - google "steam tool". Simple graphical interface - but it does it's job well. I have steam installed on a 128GB M4 & 126GB of Steam games installed... only ~4GB of that on the SSD itself.
I might of not cleared things up but I would definitely recommend the M4s, firmware flashing them is extremely easy as well. Go for a large sized drive, or two half sizes you are prepared to Raid them. Though if you honestly only want to use it as a boot drive an a handful of programs a 128 would be fine.
At current pricing - I'd go for the 256GB for convenience.
I've managed with 100-128GB boot drivers (or smaller) for at least 3 years now... if you're prepared to balance your storage, it's pretty easy
So... 128GB is definitely enough for the forseeable future - but 256GB will make your life a little simpler
Don't waste your time with raid.
As for the drives themselves... yes, M4 seems the most liked drive of those three.
The OcUK special on Samsung 830 256GB drives though is rather good! And you won't see much (if any) difference in performance. I've got one coming as a second drive - still undecided as to whether to switch boot from the 128GB M4 to the new drive or not. The Samsung may be slightly faster in general use.