It just depends on how accurate you need it to be and how much you want to spend?
I had a Garmin Edge Explore which gave good results and lots of information, but changed it to a Garmin multisport watch (Forerunner 45). It's probably not quite as accurate but that hasn't bothered me. The advantage to a watch over a dedicated unit is that it'll record heart rate at the same time, and you don't need to worry about leaving it on the bike since it's attached to your arm. The downside is that it's harder to read whilst riding and you can't get navigation on it. If you just want to record what you've done then I think it's the better option.