From our testing the two configurations we can conclude that the lap times are very similar, however the behavior of the car can be very different.
Transverse cars transfer more weight from front to rear and side to side, this allows the car to generate more traction in all directions, but as grip starts getting higher, the battle is actually to tame down steering and prevent the car from lifting the inside front tire. The car turns in harder and deeper into the corner, all while maintaining a little more rear bite.
With inline cars, the weight is more centralized and also more biased forward. This keeps the car flatter through the corners and gives a more neutral feeling with less overall traction. When bite comes up, this configuration makes the car less aggressive and easier to drive.
Now, when should I chose which you ask? There is no right answer here, however I have found that the inline car is much easier to drive over 8 minutes when the bite comes up vs the transverse car. I find myself making less small mistakes with the inline car and I can also run bigger tires without lifting and traction rolling my RR12. With that, I could say that the transverse car is probably better when the grip is anywhere from low to medium high, and beyond that, inline it is.
Hope the info helps.
CT