The XP 550 has a more advanced suspension and nicer ride. I own a 550 X2, but I have only test driven a 570, but if I had to pick I would go with the 550 like I have now. Even with both machines being EPS equipped, the 570 had more kickback on rocks and logs than my current XP. The 570 has simpler Macpherson Strut suspension while the 550XP has dual a-arm suspension with Coil over shocks.
There are other minor differences, but the specs are pretty close. both machines seem pretty equal on power from a seat of the pants feel. I actually felt that my 550 had a little more grunt than the 570, not a big difference though.
I feel the 550 is worth the extra $$$, some will argue the other way, as always, ride both and buy the one you prefer.
Mine- 2011 Sportsman X2 550 With EPS!!-Warn 2500lb Winch, Guru Brush Guard, RZR H13 bulb headlights, KC LED Yellow tint Driving Lights, Rigid light bar mounted in the pod, Autometer Phantom Oil Temperature, Coolant Temperature, and Voltmeter Gauges, Polaris Low Lock and Ride windshield, Acerbis Handguards, Symtec Heated Grips and Thumb warmer, 2+2 gallon Rotopax Fuel packs, STI HD Beadlock wheels and 27" GBC Spartacus Tires
Hers- 2012 Sportsman 550XP EPS Blue Fire LE 27" ITP Mudlites on ITP SS212 Wheels, LED headlights, Mile Marker 2500 lb winch, Polaris bodyguards