Which wheel should I buy?
Tubulars. 
Doubt it really matters - I have no experience with either but am considering the same - I was going to post a question re: 404 durability and comfort (lets see if someone pipes in here). I’m thinking this way:
Hed3’s: Bombproof almost - can ride over bumps and not worry about going out of true. I think stiffer and less comfortable (need to verify this one). Heavier. Cheaper. Likely less worry with travel damage and dropping them - can you ding those things? How do Hed3’s affect comfort vs. typical training wheels - are they a lot harsher?
404’s: Lighter. How durable?(I think should be signifcantly less than Hed3’s). Likely more comfortable (but need to check this out). More expensive. How comfortable are 404’s? How durable?
Likely very similar re: aero/speed.
Go with which one fits your bike and looks cooler…
Dave
I just purchased the Zipp 404’s and love them. I should be racing on them in one hour at the Timberman in NH but my wife and I turned the car around as we realized it was going to be a potential lightning fest with downpours all day…BUMMER.
I put them on my Cervelo P3 and the color even matches (does that make me faster?). Either way, the difference I have felt is my ability to spin and accelerate through the gears just feels quicker. I was told is has to do with the diminished drag of the wheels. The HED 3’s are a bit heavier (i think) and the five or six racers I talked to used the 404’s and loved them. By the way, I got my LBS to sell them to me for 1100.00.