Eheim 2028 if you have no expenditure limit
I bought a Rena Xp3, used it for a couple of hours, returned it, bought the eheim 2028 and I'm keeping it because:
Priming: Eheim 2028 makes life so much simpler
Media: Eheim 2028 w/media has the largest surface area for biological breakdown
Noise: Eheim 2028 makes less noise than the dripping water in my tank, while XP3 is somewhat noisier
Shaking: Eheim 2028 is very stable because it is a robust design.
Piping: Eheim Quality is a 10/10, while XP3 feels like a toy, although both do the job. Eheim has awesome connectors.
Aesthetics: Eheim feels great for a good looking living room.
Flow regulation: Eheim has a clever two way (in and out) parallel flow regulatoin. While XP3 has a knob in the outlet.
Setting up: Eheim priming took me 10 min, while XP3 took me 1 hour plus a lot of floor mopping.
Get Eheim if you plan to use this canister for 5 years. Buy XP3 if you have a good furniture to hide it beneath and you don't care about the shaking and are not trying to save space. Get Eheim for peace of mind, but don't get the one with the incorporated heaters, they have a tendency to fluctuate badly as of March 2008.