Seriously though Gaba. The difference is in the performance. The 550 is a lot lighter so it flys well on 6S batteries. With a 600 its possible to fly 6S like i do but the performance is not really there. You need to set up a 600 on 8,10 or 12S to get the same performance you get with 6S on the 550.
So for cost effective flying the 550 is much lighter on the wallet.