Great choice. 5 year warranty on the 6G's so they should follow you to a new SATA 6Gb connect and you'll get another boost. My 6G's on SATA 3Gb are very fast. 30% full they get 260MB/s Write, 265MB/s Read. Super fast 4k performance as well. No problems so far with stability.
i decided to get the 120GB 6g (200$) instead of the 115GB 3g (198$), because for a few dollars more i get a drive that could get better in future sata3 box.
In my case it was a no brainer, for you there's a 50$ difference in price between the 240GB 6g (450$) and 240GB 3g (400$)
Is 50 dollars worth it to you for a little bit of future proofing?
How much of that 120GB are you going to be using? SSD's seem to have certain performance advantages in certain allotments. Like 0-35% full yields fastest speeds 50-75% is the worst and oddly 75-95% is slightly faster than 50%. Anyway. It all depends on how many apps you need to run and all the content in /Library. The 240's are also a bit faster than the 120's of same generation. I have a ton of apps and only use 80GB of my 240GB.