Beds beds beds!

Sooo I'm buying a new mattress because ever since being pregnant I swear my back hurts like crazy! But have no idea what to go with. I just bought a memory foam cool gel matress for $330 but idk I'm not too convinced on it! I found a cheaper one for $300 that is 12 inches but has no cool gel, question is what the heck is the difference!? Please help ladies I'm so confused on which one I want more my new. One is only 8 in and I seriously am contemplating on getting the 12 in one even if it doesn't have cool gel! Help! Lol