I use a Rena 150watt Smart Heater, attached to the intake of my external Eheim filter. Extremely pleased with it. It looks good, produces an even distribution of heat throughout the water (if using with an external filter) and so far has been very reliable for me. It's also ceramic (instead of glass), and consequently very hard to break.
In response to another earlier post, the Smart Heaters DO have a internal switch-off mechanism to prevent overheating if taken out of the water (or if the water level drops whilst you are doing a water change). I still try and remember to switch it off whilst doing a water change though - but don't always remember, so this is a great safety feature. I once forgot to switch off an old glass heater when doing a water change and it overheated and cracked when the water level dropped - not a good day!
Anyway, I highly recommend the Smart Heater - it'll fit easily onto the inlet of your Rena external XP2 too.