The secret to buying a good mattress is to test it first
If you ask me there's really only one way to choose a comfortable mattress and that is to get your big butt down to the store and lay yourself all over everything they got available. Think about it, there is no way to tell how comfortable a mattress is without actually physically testing it.
It's like my Daddy used to say "buying a mattress is like choosing a girlfriend; you have to understand son, some things in this world were just made to perform better under the sheets". "Is mommy one of them"... "she used to be son, and then you came along and ruined that for us"
Anyway, that's another story altogether, if you want great mattress buying advice then here it is:
It's an investment so don't scrimp. Money spent on a quality mattress is money well spent, it will last longer, You will sleep better and you'll feel as awesome as you think you are. Don't be scared to try them all out, remember they are all different and you know what so are we! How do you think Goldilocks ended up in the perfect sack...? she tried them all out first, that's how.
Don't listen to your girlfriend, size does matter and bigger is always better. I run hot so I need a big bed to keep cool. If you're a snuggler, then just go big anyway, you'll thank me for it in the long run. Don't rely solely on the salesman - you gotta trust your instincts. When the guy asked me if I wanted memory foam, pocket springs or silicone, I simply said "yeah, sure Bob that's sounds great, I'll take them all". After a lot of confusion Bob returned with a mattress that cost £2500 and I told him to go to hell... See instincts like a killer...
Shop around; mattress prices like anything vary from store to store. Bob was right about one thing that mattress is like sleeping on a cloud... I bought the very same mattress, ex demo, from a different store with not a mark on it for only £500 and I got to take it away with me the same day. Boom - job done well.
Check out some of the great Bedroom Furniture products on Mustard Monkey.