The 6G is almost twice as fast as the 3G. What you'll notice is very subjective. Both are very fast. If you run benchmarks on both drive, yes you'll see the difference, but in real use? I couldn't really tell from moving from a SATA 3G to SATA 6GB SSD (both SandForce based).
That said, if you have a Early 2011 or later machine, get the 6G. If you have an Mid 2010 or earlier MacBook Pro with only SATA 3G, the 6G drive will run at 1.5G speeds. If you already have an SSD and are only look to upgrade, it may not really be worth it.