I just installed a set of Rigidco's rock rails, and my first impression is that they are pretty stout. They mount up fairly easy, provide good coverage since he added the skid plates to the bottom, and look really nice on the Jeep. I used them in anger for the first time this weekend, and they held up pretty good (still haven't seriously abused them though). Plus, he is local for you, so you can dodge some shipping costs.
As far as the whole gas tank skid/rock rails debate, I say rock rails. The Plastic tank on the 97+ XJs can take a fair amount of abuse, and would be a hell of a lot easier and cheaper to fix then dented rockers and doors. The best plan would be to get both though. . . .
Oh, and I wouldn't spend to much time looking for a stock gas tank skid. They are about damn near impossible to find for the newer body style, and when you do find one they usually are just as expensive as the aftermarket ones. If I were you, I would ask Terry at Rigidco if he can fab one up for you. . . .