How is the cheekweld on the fixed brace versus the one with the folding mechanism for using the factory iron sights.

My opinion, the fixed brace makes it very very hard to use irons.  That said, even the folding brace seems like you need to bear hug the gun.  Note that I am an AR guy and like lower 1/3 co-witness, so I do like more of a heads up position when I shoot.

There is middle ground with something like the KAK adapter and an AR buffer tube.  It was better than a fixed SB Tac stock but not as good as the folding SB Tac.

I had a hard time getting a good sight picture using the fixed brace, its just a tad high when trying to cheek weld. 

The folding brace is at a little better height and easier to get a good sight picture. Folding brace is much superior IMO. 

If you decide you want the fixed brace for a K model, PM me, I'll let go of mine for a decent price. 

Agree with the other opinions stated above.

I also had to do some judicious Dremeling in order to get my fixed brace to slide over the receiver while the folding brace went on just fine.

One plus for the fixed brace is that it is very solid. The folding brace feels a little more fragile, especially in the area between the rubber brace and the hinge area (where the logo is).

Fixed SB Tactical brace SUCKS for any variation of a sight picture, you won't be able to utilize the iron sights.

Opt for the folding version instead.