I have both, and for some reason (perhaps psychological only), I find the 303 Firecrest CC's feel "faster". The 404's have Conti 23 mm Grand Prix 4000SR's, glued with Vittoria mastic as per best practice according to the tubular thread.
The 303's have Conti 23mm Grand Prix 4000R's with Conti ultralite butyl tubes.

Weight of the 404's all in including zipp ti skewers : 2120 gms
Weight of the 303's all in including zipp ss skewers: 2420 gms

But.......I carry a spare tubbie plus a can of pitstop when I ride the 404's :weight 350 gms. When I ride the 303's I use a small seat bag (Lezyne) with a spare tube : weight 170 gms. I did not include Co2 inflators etc, since I carry that stuff for both.

So, the weight advantage of the 404's drops to 120 gms.

One more thing. I think that the Crr for the clinchers is much lower than the tubbies (I know.....not a fair fight since I'm using relatively high Crr tubbies.....but I don't like punctures, etc.). My guess is that the Crr advantage of the clinchers more than makes up for the slight weight advantage of the tubbies.

There have already been discussions that indicate that the new 303's and 404's are quite close aerodynamically until you get to greater than 15 deg yaw.

Comments?

side wind issues?

bearing or hub issues?

tires make a huge difference in how a bike feels as well. and the gap between clinchers and tubes is getting smaller all the time. That is probably the majority of the difference you feel.

I don't think "feel" is a very good metric for comparing wheels. CRR, aerodynamics, 100g weight differences, etc. are all important, but whether you can feel these differences at the saddle is questionable.

For example. I run my clinchers at about 115psi. However I've heard that a lower pressure (around 95-105) could actually lead to a lower crr. However, I "feel" much faster when I have a high pressure in my tires.

My guess is that the clinchers are probably faster, but the difference in feel is just tubular being more comfortable.

I absolutely agree that "feel" is a lousy indicator of performance, and in fact a great feeling wheel could easily be a worse performer than the not so good feeling wheel. Perhaps I should have changed the question of my original post. I used to be a tubbie advocate, for the feel, performance, etc. But based on numbers and very little if any weight advantage, and as an added bonus "feel", I am now swinging over to admitting that in the case of the two wheels I've compared, the clinchers are better than the tubbies.