Duat or Duats, which is better?

I have tried both, and they are more or less similar in terms of capability. It's not like one is distinctly superior to the other. You can register for both with your certificate number and it's totally free. You can also use them to file your flight plans, and I think (someone correct me if it's wrong) that they both record your weather inquiries. I suggest you tinker around a little with both sites and just see which one you like best.

https://www.duat.com/

https://www.duats.com/

I also like this one for weather since you don't have to log in:

http://www.aviationweather.gov/

A friend of mine just introduced me to this one as well, but I haven't used it too much yet:

http://www.navmonster.com/

DUAT is run by DTC and DUATS is run by Flight Prep. Both yield the same data and the only requirement to receive the data is possession of a student pilot's license (medical cert) as a minimum.

You can register thru either website with this certificate number or a pilot's certificate. I believe the only caveat is that you can only register on one of them but I'm not a hundred percent certain of that.

I use the commercial flight planning software from Flight Prep that incorporates DUATS into your plan if you so request.