I've had a Rena XP3 for 3/4 years, and I've found it to be generally very good, simple, effective, but imperfect.
The rena has a lever on top that isolates the canister from the tubes to the tank, you use the lever, take the canister away, and clean it. Great. I often find that after I do this it gurgles away for some time, I cleaned my filter today and it's been gurgling for 6 hours now I think. I refill it with tank water before putting the lid back on to try and minimise the problem, but it tends to do it anyway.
I've never owned an Eheim to compare with it, but when my Rena packs up I would certainly like to find a new external that doesn't have this problem, if I can so so without compromising the simple effectiveness of the Rena.