Wow this is quite the old thread.
DH and I currently have a queen. We started off on a full and it was just too small for either of us to be comfy. We bot sleep in kind of a side semi fetal position, sometimes more on the belly, back-to-back. That just takes up some room. Plus, he gets icky sweaty at night and I like to move far away on those nights. lol So, when we move and have a bigger master, we plan on upgrading to a king.
We love our high density foam mattress also because it limits the disruption of sleep with one is tossing and turning. It transfers movement much less than a spring mattress. So... we'll get the same mattress in a king when the time comes.