If you're looking at it from a cost standpoint, the trigger will definately cost you more. They need significantly more tank space than the smashers you've previously been told of and thus so will need a larger aquarium.
Both are messy eaters but at least you can safely add corals with the mantis. I'm slowly adding small frags to my minibow 7 too add a little color to the tank. I enjoy watching my mantis...it's really interesting to see how anal he is about closing up his hole for the night (he carefully studies every piece of sand he uses).
If you're worried about getting hit by a smasher, you might want to consider how bad it would hurt to get nipped by a trigger...they can easily take a good sized chunk out of a finger.
If you decide to go the trigger route, make sure you watch the power cords in the tank. A friend of mine had an Undulate trigger that deep cooked himself chewing through the power cord of a powerhead.
Either way you go they're both fun to watch. My wife's taking over my 90 as soon as I get my new tank. She really wants to set up a trigger tank.