I use both, and i like both of them pretty much the same, though i probably use MX more often. MX 2004 doesn't have the normal mode for actionscript where it basically creates AS for you, so it's alot harder for new flash users. Also the components have been re-vamped, and the scrollbar component is gone and has been replaced with "scrollpane". If you need to use it you'll have to copy it from MX. Also the built in effects seem pointless to me.
MX 2004 seems more stable to me, and usually for big projects i'd use 2004, and for smaller projects ill use MX. I think this has been said in another post but if u have MX, try a demo of 2004 and see if you like it. 30 days should be enough for you to decide, and if you have to opportunity to have both, i think that would be the best way to go.