I would say Sebring. Nothing really compares to the quality of that one. You can't go wrong with either, though.
If you don't like any of the DLC you can get a refund. There is a 2 hour time limit for the refund. That's game running time, not DLC useage time.
The endurance pack is quite good (good Lord, happiness that is the sound of the RSR Porsche), and Sebring is outstanding. Driving that circuit in a sim should give anyone a new respect those who do it in real life: it is bonkers, but lots of fun! The AI is very competitive and well done as well.
Despite GT3 being in other sims, I will add that Studio397's are absolutely spectacular! I'm fact, those and the Porsche GT3 Cup are among my favorites of all the cars in the sim.
I went for the endurance pack and Sebring. Sebring is amazing as are the cars. Ive already got the Porsche GT3 Cup and now find myself debating whether i spend a bit more and also get Le Mans and the GT3 challengers pack