I'd say its personal preference. Kie has said or at least mentioned that the people making top times are a mixture between automatic and manual. I personally see it as a huge headache vs minimal gains for my own shitty driving. I'd be better served learning to corner better in the first place but thats just me.
I use 99% AT. In driveclub it really does not matter, the AT is quite optimal and if you want to shortshift, you can.