Depends what you're after. The cheap bike computer is cheap, small, and who cares if anything happens to it? A phone is (or can be) pretty cheap too, but some people have very expensive ones. Anyway, the cyclocomputer will tell you your speed and how far you went, but it won't give you a map of your ride. Maybe you always do the same route and it doesn't matter, maybe you travel a lot or want to explore every road in your city by bike, and it matters. A phone can also be paired with heart and cadence and even power sensors.

Personally I don't use my phone for this, I have a Garmin watch that records a GPS track. Part of the reason is for battery life (mainly for multi-day hikes). I have no doubt my phone's GPS can record for a few hours, though, and that's enough to cover most of the rides I do.

Anyway there are a lot of choices, so probably there's a good one for everybody.