Well I had FS2k2 for a year. (bought it in late 2003) Then finally got 2004 about 5 months ago. Seeing as that I'm mainly interested in top-notch jetliner simulations and scenery/weather I had to go for 2004 to run PMDG and levelD sim. Expen$ive yes, but the quality is a big improvement over basic FS2004, unfortunately you also need a good system for playable rates. (Sometimes I have to live with jerky 8-14 fps in heavy scenery since I like my settings on high).
So I'd recommend FS2004 if you are wanting to get the topnotch payware addons (especially for the best ATP simulation addons) such as ActiveSky2004, ATR72-500, FSScene,PMDG737, LevelD767 etc although there are older versions for FS2k2 as well. Check out
www.phoenix-simulation.co.uk for excellent older addons originally for FS2k2. Also
www.simmarket.com and flight1.com still carries ActiveSky WxRe for 2002/2004 right now.
Here on simviation, there are also the free meshes I dl'd for most of the world except for my favorite spots which I bought meshes from FSGenesis.net

Waiting for the Queen, PMDG!
ega-GeForce FX5200 128mb DDR AGP8x