i just ordered a pair, and i was in the same boat as you.
im 5 10, 150 and i got the 181.
i kinda think the 171 would be a little short for you, but the 181 might be a little big.
you should probably get the 181 i think.
people were saying i could do the 191 too,but i got the 181 cause i ski in the trees alot, and the lower swing weight was appealing.
what size skis do you ski now?