The mattress sellers have the Rosetta stone in a notebook that tells them all the different names for the same mattress. If you could get a hold of that, it would be very easy to comparison shop. Otherwise you have to get it outta them with your keen negotiating skills.
Don't take any crap. Mattress salesmen for the most part are slimy worm-type troglodytes, very eager to screw you over. Many of them are rude and pushy. I walked out of more than one store because of this.
I spent about a month researching online and driving around on weekends, until I settled on my kickass Serta Perfect Night. It was expensive but I love it. You have to lay down on the mattresses to see what works for you. Most places have a 30 day exchange policy but I doubt many people actually take them up on it.