The RG550 and RG570 differ only slightly, the hardware is the same, the pickups are the same and so on. The two big differences are the fingerboard woods, rosewood on the 570 and maple on the 550 and the ways in which the pickups are mounted. The 550 has the pickups mounted on a scratchplate like a Strat and the 570 has them mounted from the front with the humbuckers in mounting rings like on a Les Paul.
The EX versions of the guitars are basically just cosmetically enhanced versions with the 550 gaining a colour matched pickguard and a rosewood fingerboard, the 570 gains fancier sharktooth inlays and both guitars get different (metallic) paintjobs, and colour matched headstocks as well.