Liner Thickness

I'm having my liner replaced for the first time and have a questions about liner thickness. The pool company that I am having replace the liner is telling me I need to do a 28/20 combo with the 28 mil being on the walls. It's a $400 add-on and I was wondering if it's really needed. I have done of searching online and the benefits seem to be that it doesn't fade as fast and is more durable. The cons I have read are that anything that will cut a 20 mil will cut a 28 and that they don't last much longer. I live is St. Louis so the pool is only open from the end of May to the end of August and it's only me, my wife and 2 year old son. We have quite a few gatherings but they are quite tame. We are also switching to a salt water system at the same time we get the liner installed so damage from chemicals should be cut back.