Don Bennett:

I have ridden most of the wheels you refer to and I still think that Zipps are the best. I ride 404 clinchers because I can't be bothered with the hassle of tubs. Also I don't believe that you should only use them for racing. I use for all riding, in all weathers, in all terrains and they are superb. Go for 404's. You will never look back.

I also went with Zipps, although I chose 303's.  The owner of my LBS rides with Zipps on his bikes.  He carries, or can get virtually every brand of wheels/rims.  The fact that he uses Zipps tells me something. 

 

I have a set of each (303 tubs and 404 clinchers), both are great wheels and it very much depends on what sort of riding you do. To be honest I don't find the 404's that much quicker, but doing some triathlons it just seems right. Ordinary road riding I think (IMO) the 303's look better on my SLC-SL and means that the overall weight is very low, spin up quickly and when I am struggling in the hills I simply telll myself that I have as much 'bike advantage' as I am likley to have so there are no excuses just get on pedal harder.

Either wheelset will be find. 

 

 

 

I'd say 303s offer an advantage going uphill, but 404s have the lead the rest of the time. So unless you're planning a lot of climbing and not much descending, 404s have it. That said, I'd go for Boras, or Corima, Reynolds or Edge rims built around my choice of hub before thinking about Zipps.

 I train with a pair of Easton Ascent II aluminum clinchers, which is consider to be a climbing wheel because of it's low weight. (2120g total with tubes and rubino pro tires).

For racing I use a pair of Easton Tempest II tubular wheels (Non dimpled Zipp 404 rims laced to Easton hubs) with continental sprinter tires and they weigh less at 1935g total. These wheels are fine in cross winds and pick up my mph 1.5 and dropped 1:30 off my 20k time. 

Both of these wheels use the same R3 hub and my personal opinion is the Easton hub is better than the Zipp. Stiff wheels, never broke a spoke, easy to service myself and they roll forever.