I think I prefer the first version. At least the conflict is obvious.

Developer A creates migration 20100604181252. Developer B creates migration 20100604181253. Developer B doesn't notice Developer A's migration: there's no way to know from the timestamps that there might be a conflict. Everything breaks.

vs

Developer A creates migration 201. Developer B creates migration 201. Developer B gets a handy notification that something has happened in the meantime, and gets a chance to patch the migration if necessary.