Is most memory foam created equal and/or which memory foam mattress is best? Help!?

Please help!My boyfriend and i have been looking for a new mattress and are overwhelmed at the numerous types to chose from & we can't make a decision.The memory foam mattresses seemed to be the most comfy when we tested them out in store.But there are way too many different brands to choose from-for example we tried memory foam from tempurpedic,restonic,sealy,serta, comforpedic and more.They all seem to feel relatively the same but that doesn't mean they're all the same good quality.I hesitate 2 ask salesman's views cause they work on commission-ulterior motive... We can't afford the tempurpedic.R there any other memory foam mattress brands/types that are close to tempurpedic quality but with a lower price tag?Also, I am extremely frugal (cheap really),and I keep telling my boyfriend we should just get the 8 inch memory foam mattress from Wal Mart for less than $300.Anyone know if this one is decent quality?I think there's a 5 year warranty on it &that the brand is Spa Sensations.Thnx