i have both the xp3 and a fluval 304 and both are good. i really haven't given the xp3 a good test but here's my overall impression:
the 304 is quieter and is better constructed. the water tubes are more flexible, but much harder to clean. 304 also has a much better a water output nozzle. easier to disconnect when you need to clean or to temporarily stop the water flow. priming is harder, but once you figure out the best way for you to do it, its easy. mine also leaks alittle bit and i have to reseat the motor section (which is a pain if the filter is primed)
xp3 feels abit cheap, but has a nice clear case. huge media baskets and probably a better water flow design inside the filter. clear tubes also make it obvious when to clean. priming is brilliant. i do not like the water output nozzle/valve design at all. also seems to cycle less gallons than my 304
btw, i will probably move my message into the equipment board later. maybe a dedicated filter board...