By moving from a road bike, I suspect that you will feel the weight difference regardless of which type of mountain bike you choose. If the difference between a more XC set up and a an AM set up is only a few pounds, I wouldn't let that sway your decision. Your body will adapt. What is likely to kill your endurance more, in my opinion, is the technical aspect of a trail which involves more of your body than a comparable road ride. Again, you will adapt to this, but considering a bike that will help reduce some of that impact may speed that process up a bit.
If you work in a shop, do you have the opportunity to test ride some of these bikes on a short course or trail? That might help you to be more comfortable with how the heavier bike will respond under you and if weight will truly be a factor.