Personally, I don't like the FMS or Kenny Brown ones since they aren't long enough. If you've got a little more money to spend, you might look into the Griggs in the floor SFC . Or as an intermediate step, you could go with Global West's supplemental kit.
Some people make a big deal out of round vs. square SFCs.
I'd bet that you'd be hard pressed to measure a difference. Ultimately, the skill of the installer will have a bigger impact on the results. This way, you should get some SFCs that fit your car perfectly. As a side benefit, this will let you establish a relationship with the fabricator. A few years years down the road you may be so far gone that you'll want a full SCCA roll cage.