2nd Restonic bed and same problem!

About 3 or 4 years ago we upgraded to a king size Restonic mattress. At first it was wonderful! However, over the course of a year the mattress began sagging horribly (about 2 1/2 -3 inches). Weekends Only did a wonderful job of getting us a new bed. Well, here we are a year later and the mattress is sagging again (a little over 2 inches). First of all, I think anything over an inch is extremely uncomfortable, so having 2 inches as a standard seems crazy to me. I am 115 lbs. and my husband is 180 lbs. so we are not putting a lot of strain on the mattress. Once again I submitted all the info to Weekends Only and we have a credit for a new mattress. The issues is I DO NOT want another Restonic after the 2 beds have had the same issues. I am so happy the Restonic does a great job of honoring their warranty, however it is a huge inconvenience and we have been uncomfortable for quite a while. Our problem is Weekends Only has a limited selection of mattress manufactures so our choices are very limited. I wish we could just get our money back so we could go elsewhere and have more mattress choices. I am so disappointed and am at a loss of what to replace our mattress with.