What can I finish my windowsills with that will resist the claw marks?
Declaw your animal.
If I lived in the city I would, but I have dogs and I live in the country with alot of wild animals around.
You can get claw covers at pet stores. OR< you could apply some fabric or shelf liner to the sills using a glue or mod podge so that it is removed easily in the future.
You might have some luck with laminate (such as Formica) cut to fit on your sills. You can buy it at places like Home Depot and Lowes, or online in many colors and finishes, It's not that hard to cut, and you could use a non-permanent method of adhering it so that the sills could be returned to original if you move.
3 M Command adhesive strips. I used them to hang some aluminum pieces on my wall, but they are thin and would probably work for sills also. Double sided tape would work, but it's a pain to remove.
For the sills my girlfriends cat likes to use, she went to a stoneyard and purchased remnant bits and for a nominal fee, had cur to her specifications and polished . Since they are heavy, she didn't even have to glue them down.