The really fun part of the 747 - what sets it apart from most other planes - is being upstairs. That's Business Class on UA, so that's what you should nook.
As stated upthread, the 747-8 is a new plane and will be around for many years to come. Also, if it is upstairs you want, try the A380. With that some operators have economy upstairs - BA and MH spring to mind but I'm sure there are many others.