Serta iSeries NXG 1500 and 3500 vs Perfect Sleeper Pro

I currently have a Serta iSeries 2.0 2000 Luxury Firm and love it. It is firm and supportive. Any insight into the current iSeries line and difference between the NXG 1500 and 3500 in firm? And how would the NXG compare to the latex in the Perfect Sleeper? I got a quote for the 3500 in Queen size for $1,900 before tax, is that a good deal?

I am looking for something that I will not sink into and that will nto sag over time.