King.
Neither of us like to be touched when we sleep - he twitches once he's asleep if I'm touching him, and I just can't get comfortable until I'm back to the sweet isolation of my side.
And we can't share bedding either - After 2 weeks of me trying to yank the super-king duvet from around his tightly-cocooned and very asleep body I gave up and got my old single duvet off the spare bed. 14 years later that's still how we sleep :)
Basically, he was used to a Queen all by himself, and I was used to a Double (what you call a Full?) - so it really had to be a King for us both to have a chance of sharing well. Traveling, we share smaller beds just fine, so long as the mattresses don't have a lot of roll-together!
Downsides to a King are, as already mentioned, the price of the linen, the huge hassle of shifting it, and the sheer space it takes up in the bedroom. We hardly ever shift house. We use our linen to death (just have had to replace some, after 14 years), and have only bought houses with a master bedroom big enough for the bed (we've just shifted into our 4th house since getting married).
There is no way we'd ever permanently downgrade, although I totally agree with the poster upthread who said what a huge difference the quality of base and mattress make. We got rid of our first King bed about 3 years ago (mold. gross), and switched from a box-base to a slat-base with good mattress. Hugely better sleep as a result, and way less cross-disturbance when one partner decides to roll over by launching himself in the air, spinning, and crashing back into the mattress, while curling his duvet around him. For instance.