We've not had any incidents...

I'd say magnetic clasp would be safest. Inspired by Finn say that the lighter the amber, the more salicylic acid it contains, so they say they're the most effective.

Also I think to get one that isn't too big for bub. The string can stretch a bit over time, and you really don't want it to be getting caught on things - even though that's what the magnetic clasp is for, you don't want to have to test it.