UT2003Demo, no contest. Consider:
-UnrealTournament usually has over 2000 DM servers available at any one time.
-There are certain extremely popular public UT servers that are normally hard to get into because the server is normally full.
-Example: The Factory public TDM server. This server is set up for 6 on 6 TDM, and normally is hard to get into. Usually, I'd have to connect as spectator and watch for someone to leave, or try to beat someone in as the server reset for the next map load.
-UT2003Demo only has 2 DM/TDM maps, 1 CTF map, and 1 BR (Bombing Run) map, along with one basic skin (multiple faces), and doesn't have some of the weapons (mainly the Redeemer and the Ion designator thingamabob).
-On the night that the UT2003 demo was made available, the UnrealTournament servers were largely abandoned, as people overloaded UT2003Demo download mirrors attempting to get the new game.
-On that same night, over 500 DM servers sprang up within hours of the download's availability.
-By the evening of the following day, there were 1400+ DM / TDM servers.
-Within 3 days of the download's availability, there were 1700-1900 DM/TDM servers available for the Demo.
-Predictably, many popular UT public servers are now empty, or nearly empty. This includes the Factory public server.
-UT's graphics, while based on an aging version of the engine, is to the point that most hardware can / does run the game fluidly in most situations.
-UT2003 Demo graphics, based on a new version of the engine, is extremely gorgeous, but taxes hardware, and especially the CPU (during gameplay), to the point that minimum framerates for most people are at best in the mid 40's, with most probably falling somewhere in the lower to mid 30's. Translation, extremely hardware intensive, while most people using the demo don't have hardware capable of giving the same fluidity that UT has.
Conclusion: Any game (demo!) that can grab the majority of the fanbase for it's extremely popular predecessor, despite being extremely feature limited and hardware intensive has got GoTY written all over it.
I still find it hard to believe that there are nearly as many DM/TDM servers for the 2 map demo of ut2003 as there are for the full, mature, well-established UT.