The 440s are as good as most other companies deep carbon rims and will be great. For the 404’s Zipp employed a new construction technique inproving aerodynamics while saving weight. Go to the zipp website and you’ll see how the rim has a shape to it rather than going straight like the cross section of a cone.
I have both front wheels. A 2002 404 and a ??? 440. The 404 is about 100 grams lighter than the 440. The 440 requires carbon brake pads, the 404 doesn’t. The finish on the 404 is definitely better than the 440. But when it comes down to it you can get a set of 440’s for about 1/3 to 1/4 the cost of new 404’s, so… I you spent that money on some other go fast piece of gear or training and bought the 440’s, I’m quite sure you would come out ahead at the finish line.
I have one of each and love my old 440. It is from around 1992-4 and is the lightest of the 5 race wheels I own. With a tubular it’s 50g heavier than my 404 which is sans tire.
Buy the 440’s, save some dough, take the wife and/or girlfriend out for a nice dinner for putting up with all the training and put the rest into your IRA.