If you haven't bought any of the SSD's yet I would purchase the 512. Not only can you use trim, but if you were to RAID0 them, you would effectively double your chances of losing all of your data (all things being equal). Because all it takes is for one of the two drives to go bad to lose ALL the data on both.
If we were talking HDD's I might consider the RAID0 simply for the performance advantage. But with SATA3 SSD's you're not going to see a performance advantage with RAID0.
By the way, I have two 120 GB SSD's myself and they are in RAID0 and am happy with the setup. The reason I went with this option is because at the time I built my PC, I could only afford one 120 drive and not a 240 drive. Also, when I bought the second drive, I reinstalled Windows and chose to RAID0 just so that it would appear as 1 drive to Windows and not worry about 1 getting almost full and trying to balance what to put on each. This is another reason I say go for the 1 drive.