The boat ride is quicker, but monorails run more frequently. If it looks like a boat is heading back to MK, get on that. If it looks like it just left, take a monorail.
As the PP said, there is no official grace period. Disney recommends you show up 15 minutes prior to your ADR time. Technically, they can turn you away if you check in a minute past your ADR time (have seen it happen before).