20 mil should be OK, It makes installation easier and will stretch in very nice. The 30 mil is thick and it is alot harder to get the wrinkles out when you are installing it, Expecially if you have vinyl steps.

I beleive that you wont really get much more life out of a 30 mil vs. a 20 mil. The life of your liner really is effected more by your chemical maintanance.

Hope that helped.
Nick