What are you running the 600 to? I have a Ti600.2 and 400.2 and ran 0 gauge in my old install because I had the 600 at 2 ohms bridged on 2 12's ported and it pulled alot of current.
Then it depends on how you like to listen to your music. I will be using 4 gauge to run my 600 and 400 but I will never push the system daily so it will be ok. If your going to run it hard I would use 0 gauge. That 600 will run warm and pull a good bit of current at 2 ohms bridged while pushed hard.