I think remembering a combination is easier than worrying about misplacing your key.

Combination is probably better, but I think you’ll find that you hardly get any use out of a lock anyway. Most places don’t have lockable lockers. But maybe those few times you use it makes it worth it, personal preference I guess.

I’ve found that most places that do have lockable lockers offer lock rentals. That way you know that they are going to fit the locker and it’s usually a euro or two that you get back when you return it.

If you buy any kind of lock in the US and you’re thinking of using it on lockers in Europe, make sure you get the smallest lock you can find; I spent like 10 bucks on a really nice combination lock before I went to Sweden a few yrs ago, and ended up just ditching it because it was too big to fit through the loops on any locker I saw there.