XFCE usually. Two years ago it didn't support my weird multi monitor configuration, so I still have one computer with Gnome.

There are two wolves inside of me. I'm at a furry convention. Send help.

It depends! Most AC to DC converters actually convert from AC to DC back to AC (at a higher frequency) then to DC again. The higher frequency means you can use a much smaller and lighter transformer for the same power. Depending on how it does the first AC to DC step, you could be ok giving it DC, but I wouldn't do it unless the power supply says it's rated for it.