Definitely go for the bigger filter. I have used Eheim filters on all my large tanks and always overbuy from what they say you need. The advantage I find of a canister filter is that you can just disconnect it (make sure you get connectors that allow you to do this), carry it into the bathroom and empty it into the bath. No mess, no fuss. It saves all the fiddling about inside the tank which my arms aren't long enough to do easily without getting soaked. Plus you only have to rinse the bath down afterwards to clean up
