Rule of thumb for wheelers are 1.5x vehicle weight. Even with that ratio, we had one recover where we had two 9k winches with blocks for 36k pulling power and still couldn't move a 4k jeep miled in mud. Wasn't until we put a 3rd 9k winch on that we were able to get him out of the thick goo which was beyond his frame.
It is highly unlike though that you will ever get into that type of situation with a TC. I had a 9k winch left over from my jeeping days which I mounted on the front of my truck. It won't get me out of everything, but should get me out of anything I happen to get myself into as long as I'm not thinking of using the winch to get through it. Say dropping a tire into a stump hole or not being able to get enough traction to climb back up a rough road.
Don't forget, all of your support equipment should be rated to handle the max load of the winch. This means heavier tow straps, heavier shackle, snatch block etc for a 15k winch which means more weight and more expense. Just one of the advantages of staying with a 9.5 winch or smaller. For support equipment, recommend:
1 snatch block rated for 2x the winch
1 tree saver (3" strap) long enough to reach around the normal tree size and rated 2x the winch
1 (prefer 2) tow strap rated for at least 1x the winch to extend the cable reach
1 shackle rated for 2x the winch (for connecting the snatch block to the tree saver
1 shackle rated for 1x the winch for each strap
If you don't have any experience, make sure you get some training from friends or a local jeep club on how to properly use it. It could make the difference between a successful recovery and getting into more trouble.
If your worried about a smaller 9.5k winch being under powered, don't as even a 15k winch will be under powered at some point. Plus, I have pulled my 7k truck out of the mud stuck to the axles with a 1.5k come-a-long and a little help from spinning the wheels.