Typically RTJ flange is to be mostly used for #900 and higher piping calsses...instead of FF and RF ones to provide a better seal against flange leakage...

Any flange combination gives a total sealing as a combination of geometry of the flange and the geometry and the exact and correct deformation of the (elastic) sealing material, and how well the elastic material then can withstand pressure and the chemical and temperature environment.

A correct RTJ sealing will have better properties than a RF og FF sealing. The rest is a question about lifetime, cost, and consequences of failure. No 'correct' answer, but the larger consequences or likeliness of unwanted failure, or the higher quality wished for, the better to go for a RTJ solution.

Any solution required to be certified for 'sour service' needs to qualify for a range of conditions that can be difficult to pinpoint to absolutely certain parameters over time. Why not RTJ? Is extra cost not comparable to risk/lifetime, or will it give supplying problems?