First-time clipless user, I don't have knee issues --but did when I was younger/played basketball.

I'm not too concerned about the knee, and it probably won't even be an issue. It was more of a youth/growth issue.

Suggestions please.

I have both and I prefer the X. I had to start using the Zero because the inside of my heel would touch the crank as I pedal and ware out a hold in my shoe.
I prefer the X because the float is much smoother, and the cleats last longer and more resistant to dirt problems..

I prefer the Zero because I don't like the excessive float of the X. BTW, sorebutt, I'm curious as to why you say the X cleats are more resistant to contamination. I think most people who have had experience with both would say it is the other way around.

The Zero is a sweet pedal as you can adjust the float to your liking. I have never tried the X but I am happy Zero user.

I agree, I like the ability to dial in just the amount of float I want on the Zeroes. I ride on rollers as well, and too much float becomes a pain when releasing from the pedals.

As for dirt, you really need to buy "Cafe Covers" for walking around. I also note that the Zero cleat is very slippery on tile or any other smooth, hard surface.

That's one reason I love my ultegras... they work pretty well on ice and slush, which I'm forced to ride in lately.

I went ahead and ordered the Zero's.

Have used X-2 for 5 years - no problems. Much nicer than Looks with ARC cleats. I don't have knee problems. The free floating of the X series was a little weird to get use to at first, - sort of like standing on ice cubes.

Zeroes and for those who walk with them get the coffee shop covers.

Yep... x-5 user here... and have coffee cup covers for them

I currently use the Zero's and they are my first clipless pedals. I like them a lot because I can adjust the float. Not hard to get in or out of. I'd buy them again. Coffee shop covers are definitely worth considering if you will be walking in them.