I do not confess to knowing much or anything about this particular topic, but my instinct would be to measure the gearbox temp. I would imagine that will run potentially hotter due to more moving parts and heat transfer from the block. They also tend to be the more expensive part to replace/repair out of the two. So I would rather know if that was going wrong if I had to pick between the two. In an ideal world you would want to measure both. Especially for a track car I would assume. Do they need close monitoring?