also check out the extrawheel if you are looking at getting a trailer. They are (reputedly) fine to use off-road.
http://www.extrawheel.com/
Canal tow paths most trailers will be fine.
If you start going much rougher surfaces though they start to get very compromised.
With my old trailer ruts and any obstacle large enough to require you to pop up the front wheel were the biggest problems.
They're also a fair bit more weight compared to panniers, is another moving part that can fail and you can feel the drag.
Don't get me wrong if you load up any bike in any way with 12kg of tent, bag etc you'll feel it.
But for me i just found the trailer to be more of a hindrance than a help so ended up giving it to a mate.
Off-road on fire trails and bridleways it was an absolute nightmare, throw in mud and i was cursing like a drunk pirate.
On road i really felt the extra drag compared to panniers, wasn't so bad on the flat but up and down hills i just didn't like the feel at all.
If your thinking about a trailer your best bet is to try on out before handing over your hard earned as i'm certain if i'd have tried my old one loaded i would not have bothered.
That's just me for the riding i did/do mind.