Go to your local library and check out Consumer Reports to find the best,
longest lasting deck treatment. I have had good luck with Cabots pigmented
products. The transparent and semitransparent stains don't last nearly as
well. Over time, PT wood will tend to warp and get splintery, and finally
rot, and deck treatments tend to slow the process, and greatly increase deck
life.
Roger

Just to add my 2 cents worth. I'm not sure how good modern pressure
treated wood is. I know they are switching from arsenic to copper
treatments, but I have pressure treated stairs on slopes, in the
ground, that are over 25 year old.
Also, last week at our swim club's work party, I applied Thompson's
water seal to picnic tables and it was awful. Stuff was crystallizing
out of it clogging sprayer and when painted on thick was also phase
separating. Probably related to reformulating for VOC's but my point
is you need to shop carefully and be up to date on available products.

Frank