How much have you got to spend????
The aggresor has the following Features: Semi-Auto Action, Rear Cocking Venturi Bolt, Quick Disconnect Pin, Sight Rail with Beaver Tail, Bottomline Setup with Steel Braided Hose, Two Finger Trigger, Anti-Double Feed, 45 Grip with Rubber Cover, 9 inch Micro Honed Muzzle Break Barrel, Tournament Level Velocity Adjuster, Vertical Feed Aluminum Body, Aggressor Drop Forward and 2 Tone Anodizing.
The xtra has the following features Semi Auto Action, Straight Feed System, Rear Cocking 2nd generation Venturi Bolt, Field Strippable T-Pin, Sight Rail with beavertail, Bottom-line set-up with micro-line hose, 2 Finger tigger with regulation guard, Anti-double feed, Filter system, 12"micro-polished barrel, Aluminum 45 trigger grip frame, Low pressure chamber, TL velocity adjuster, Removeable gas-thru foregrip, Mini expansion chamber, Newly designed cocking grip, Special anodizing, Deluxe drop forward.
As you can see the Xtra has more features then the agressor so i would go for that but if you have the money to buy one of these new you can get a good deal on a second hand marker. £200 can get you a 2nd hand cocker these days