This pair is less about torque than it looks, because underneath they are the same base. The CSL DD and the GT DD Pro share a chassis; what separates them is the power supply and the licensing. The GT DD Pro ships with the bigger PSU for 8 Nm and, more importantly, the official Sony licence with Gran Turismo 7 firmware baked in. It was developed with Polyphony Digital specifically for GT7, and that licence is the whole reason it exists - it is the only direct drive wheelbase that works on PS5 at all.
So platform decides this one before torque does. If you are a PlayStation racer, the GT DD Pro is not really competing with the CSL DD - it is competing with nothing, because it is the only officially licensed DD option on the console. Pay the $599 and you are in. The CSL DD, by contrast, is the PC-and-Xbox value entry: $399 for the same underlying base, minus the PS licence and shipping with the smaller PSU at 5 Nm. Most reviewers will tell you that on the 5 Nm base the Boost Kit is the upgrade to get - the jump in detail and authority is bigger than the price difference suggests - which brings it up to 8 Nm and closes the physical gap to the GT DD Pro.
Put simply: buy the GT DD Pro if you need PlayStation support, and take the CSL DD if you are on PC or Xbox and want the cheaper route in. Just go in knowing the CSL DD's 5 Nm is the starting point, not the ceiling - add the Boost Kit when the budget allows and you have effectively the same 8 Nm hardware as the GT DD Pro, without paying for a console licence you would not use.