I've taken 3 turkeys with the x-bow. I think I used 3 different broadheads - 1 fixed, and 2 mechanicals. The one that seemed to be most effective (though I'm sure shot placement had something to do with it as well) was also the cheapest - Cabela's Lazer strike mechanicals. I used NAP Thunderheads for one and they did okay, but blew right through the bird like it wasn't there.
If I was going out again I would use a mechanical with a large cutting diameter (minimum 1.5"). It doesn't have to be pricey either- you are shooting a smallish bird, not a 200 pound buck. I would find a cheap, 3 blade mechanical with at least 1.5" diameter cutting blades that flies well in your x-bow.
Accuracy is more important- shot placement is real tough with a turkey.