These are very distinct fragrances. I would never think to directly compare them, apart from describing them against each other since there's such a contrast. Chanel Sycomore is a smoke-touched rich vetiver, with a hint of sandalwood in the base. It's mysterious autumn. Terre d'Hermes is citrus, cedar, and flint, with a bit of vetiver. It's a summer day stretching from mimosas to a warm evening rain. Entirely different experiences, both.
If you like both, both are worth owning. However, Sycomore gets the edge as being one of the very best fragrances for men (or adopted by men) out there. A must, in my opinion.
The two have a vetiver note in common, although it's much less prominent in TdH. I prefer TdH by quite a lot, but that's mainly because I'm not a big fan of vetiver. If you love vetiver, the odds are good that you'll enjoy Sycomore a lot.
As others were saying, TdH is a citrus with a mineral background, Sycomore is a big vetiver (fresh/grassy) with touches of wood and smoke. I am a big fan of Sycomore. But they are very different and there is space for both.
Sycomore is a masterpiece in its genre (woody-vetiver with just a tad of incense), TdH is an unpretentious, yet good smelling concoction of grapefruit and geranium on a strong synthetic base. The main difference bewteen the two is that Sycomore is worth owning, TdH no.