Oh, okay, I think I understand what you are saying. If I were a purist and wanted to use a manual shifter for the majority of the cars in the game (like they would have in real life), I would only be able to upgrade to the Fully Manual Transmission for those cars. But if also wanted to be a purist and use the flappy paddles for the F1 cars and the modern sports cars (probably most Gr.1 and likely some Gr.2 and Gr.3 cars as well), I would upgrade to the racing transmission and use the flappy paddles on the wheel.
However, I'm not a purist... I'm someone who wants to use the best tool for the job and because this is a video game I'm going to work within the confines of what I'm allowed to use in the video game. That said, I haven't noticed much of any difference between the Fully Manual transmission and the Racing transmission while being exclusively on a flappy paddle wheel setup. Both work about equally in my book. Like I said, I have only noticed 1 car to have a noticeable change between the Manual and racing transmissions. I have A/B tested the 2 transmissions with a number of cars and they feel and react basically the same. If I notice one feels better than the other, I generally go with the better one for the job but it's still a toss-up on what other things can benefit from the extra pp you gain from using the manual transmission. I'm pretty sure I use the manual transmission with the 787B because I could add more front-end downforce and decrease the ballast giving the car better overall handling characteristics (for my driving style), better acceleration and better braking performance. These 3 things far outweigh any benefits the racing transmission gave over the manual transmission.
I suppose the purist would also believe that the 787B never came with a flappy paddle shifter and therefore should only get the manual transmission put in it instead of a racing transmission even if I don't have a manual shifter setup as part of my sim rig.