you always have to take a nice bouncing leap onto the bed.
I would think going with a memory foam topper would allow the mattress to last longer. Because you can replace the worn foam. My aunt did that, and her bed is ages old. Of course she can get any foam she wants to fit her needs.
We just went shopping for a new one too. Went close to memorial day, which gave some added savings.
My aunt wanted to spend little as possible, and i wanted something soft and squishy as possible. Since i might be the most frequent user of the bed, well at least the first. Shopped at a few stores, checked out department store. Found one alright mattress at mattress giant for 399 (full size). Final stop of the day was a mattress firm clearance store. Now she wasn't looking to purchase that day, since we had like 3 more days till moving in the house. But the guy showed us their brand mattress, said it was 450+ or something. My aunt said that Giant had a similar bed for 399. So the guy price matched, knocked an extra ten off and threw in free a bed frame (which was needed). My aunt bought it right there.
It's a pillow top, and it does have the outer rim spring to keep the sides from sagging. I didn't notice them sitting on the edge, but my aunt did. And you don't notice at all laying down. After getting it home, my aunt thought it was a great mattress and that i picked out a good one. I haven't had any complaints and love sleeping on it. Fits the bill as soft and squishy for me.
I would suggest shopping now, getting prices, and then buy on the july 4th weekend when all the stores will have a sale. And don't be afraid to price match on different brands, especially the store brand models. Each store will think theirs is the best in comparrison.