Still not convinced on the longevity of a memory foam mattress - how do they compare to the old style spring mattresses?

Not much of a choice in beds where we are at the moment, but maybe someone could also point/comment about these 2 options we are toying with at the moment:

Memory Foam bed from Therapedic
• 3" visco, memory foam
• 6" high-density poly base
• .75" super-soft foam, 1" quiltflex, 1.25 oz. Dacron

vs

the Innergy Therapedic one:
• Exclusive alternating coil design
• Foam-encased coil unit
• Polyurethane insulation foam
• Pillow-top cover quilted to 3 inches of pure latex foam rubber
 

Both a medium and we need to chose one soon...any advise?

thank you in advance