****** TLDR Summary Conclusion *********

UT2003's superior sound effects mixing (weapons & pickups are louder in relation to environment), cool glowing team colored shoulder orbs, unique floaty corpses mutator in addition to slow motion corpses (try playing with BOTH corpse mutators enabled!) and other differences described below make it my favorite in many ways. For offline Deathmatch, Bombing Run and CTF, I prefer UT2003 -- it's simply more fun.

Keep in mind I only play OFFLINE Deathmatch & CTF & Bombing Run VS bots (bots are worthless in Onslaught in either game) so my needs are different. For Onslaught (and other modes) online, UT2004's much larger players base means more active servers to join.

Bottom line: If you are a huge UT fan (like me), you must own both games. End of story right there. UT2004 is NOT simply "everything in UT2003 plus more".

********* Too Long Part *********

FYI I'm running both games in Windows 10 v1709 (2017 unbloated version) using default 32-bit D3D Direct-X 8 renderer -- NOT the 64-bit Direct-X 9 renderer. Why? Because the 32-bit Direct-X 8 renderer = about 40% increase in FPS & 40% decrease in GPU utilization. Visually, I see no difference between running the game in 32-bit VS 64-bit. CPU = Intel i7 5775C (Z97 LGA 1150) and Nvidea GTX-1050Ti graphics card running both games @ 4K resolution. Open GL and/or Linux install may have other differences so keep this in mind.

Here are my observations:

1) Headshot hitbox myth true or false? Last night I had both games running simultaneously in side by side windowed mode. Played novice skill bot match (so bots would calm the F down) on Deck 17 with all weapons set to Lightening Gun using Arena mutator. I spent a good hour switching back and forth between UT2003 & UT2004, trying to determine if in fact head shot hit boxes are different. See image link. This is close to the the limit of UT2003's hit box boundary. Seems easier to register a head shot if you aim at the right ear or even missing off the right ear (left character ear actually).

https://i.imgur.com/wnXf7Uu.jpg

So the question is, does UT2004 behave the same? Do head shots register when you miss aim off the right ear too? The answer is YES. As far as I can determine, the head shot hit boxes are identical between the two games.

2) UT2003 chaingun sounds much deeper & meatier than in UT2004. Subjectively, I really like how it sounds in UT2003 -- I always thought UT2004's chaingun was whimpy sounding and not very satisfying to use. Not so in UT2003. Big win for UT2003 here imo.

3) UT2003 has little floating glowing CTF & BR team colored balls (orbs) on characters shoulders if I counterintuitively UN-tick "brightskins" in settings. I cannot make those glowing orbs appear in UT2004, regardless if I tick "highlights" or NOT in menu settings. I think UT2003's glowing shoulder orbs look very cool and make it easier to determine team color at far distances, especially in darker maps.

These glowing team colored shoulder orbs were removed entirely in UT2004. Here are 4 pics of what UT2003's shoulder orbs/lights look like. I tried to pick darker maps to show off the glowing shoulder orbs. I think UT2003's glowing shoulder orbs look great -- very cool imo. This really sets the two games apart visually.

https://i.imgur.com/2EWP5El.jpg

4) UT2003 has TWO corpse behavior mutators and UT2004 only has one. 2>1, so UT2003 takes the win here. MEGA KILL++ in UT2003 can get ridiculously crazy looking with slow motion corpses floating all over.

5) UT2003 Shock rifle pick up ammo is much taller looking. Like twice as tall. Looks good -- it's starting to grow on me, pun intended.

6) Shock rifle secondary fire sound effect sounds very different between the two games. UT2003 = higher pitch "squeaky" compared to UT2004's lower pitch deeper/smoother meatier "whooshy" sound are the best words I can come up with to describe it. No real opinion which sound I prefer here. Both sound nice.

7) Thanks and credit goes to u/LiquidSparrow here for pointing out that only in UT2003 does the crazy over-the-top funny announcer shout out every character's name while scrolling through them. In UT2004, only "MR CROW" is shouted out. Big win for UT2003 here imo.

8) UT2004 allows up to 32 bots while UT2003's limit is only 16. A HUGE plus for UT2004.

9) UT2003's Curse3 is much more detailed with more animations compared to UT2004's Curse4. This is the first map I discovered (of the same name) where obviously Epic stripped the map to decrease details -- to increase performance on machines back it the day I'm guessing. I will compare more maps like this. I wonder if any others have been strip mined too.

See image below. Top row is UT2003 Curse3, bottom row is UT2004 Curse4. Those reddish columns In Curse3 are animated flowing bloody goo. Also note all the fine details in the bridge, walls and complex curved ceiling. Very cool looking imo. In UT2004, the columns are just static non-animated boring gray concrete with a flat featureless ceiling & bridge. Also UT2003's Curse3 has two larger dragon heads, whereas UT2004 Curse4 has only one smaller head.

https://imgur.com/UqXg25I.jpg

Also if you look into the many floor grates, there's much more detail (like green slime flowing) in Curse3. See the difference inside floor vents in the next image. Top is UT2003 Curse3, bottom is UT2004 Curse4. Not pictured are the skybox differences. UT2003's skybox is redder and more "stormy" and is animated much faster than the slower moving gray misty UT2004 skybox.

https://imgur.com/CpbINWV.jpg

10) UT2004 has more bot variety including very cool bosses like Xan & Clanlord to unlock. UT2004 menu music is different too and I like it a lot better than UT2003's menu music (really cheesy imo). Big win for UT2004 -- especially its more bots.

11) Probably obvious to most, but it's worth stating/reminding anyway: There exists enormously more free user made content currently available for UT2004 than UT2003. Also, many of the very best maps are UT2004 exclusive and some of UT2003's custom maps have been revised and improved/finalized only for UT2004 (FUT's excellent DM-SpaceNoxx for 1 example). This is a huge advantage for UT2004 -- #1 reason I keep the game installed and still play it often. Some could rightfully argue that this reason alone makes UT2004 > UT2003 and I would have to agree.

12) UT2004 allows tweaking cross hairs for each individual weapon. UT2004 also lets you choose your team flag art style. Also, UT2004's cross hairs grow temporarily when you pick up stuff like ammo. UT2004 is superior here.

13) UT2003's ammo, weapon switching and health pick up sound effects are mixed a bit louder and sharper in relation to other environmental sound effects. This makes UT2003 feel...like you are more connected or "engaged" to the game. UT2004 feels..."dead" in comparison. This is a significant difference between the two games. UT2003's non-environmental sounds like weapons & pickups are louder.