If all you ride is mountains the lower the better. What do you have for gears? I try to gear in the 2.2 range. But I cannot go any lower than I am at 19/43 with my 8 tooth 3 pitch. No smaller than 19 on the top and 43 is the biggest on the bottom. You must watch chain tension with the 43. I want to build a bigger case or use a doo case.
I agree also with the 19-42, I am running that in a viper with 7 tooth 3-0 and 162" track, works great, probably works even better with a 153. I have an 18 tooth in a 500 mountain max and chain wrap doesn't worry me too much on that sled because of the lower horsepower. With the 19-42 you can still use your 70t chain