I've used pea gravel and it was a nightmare. In the winter, the Newfs would lie in the snow, it would melt and then the gravel would freeze to their feathering. They'd carry the pea gravel inside, it would melt, and then I was left cleaning up pea gravel off my floors.
We tried grass for a short period of time. Huge regret. When big dogs pee it causes a puddle or river. The dogs would either step in the pee-puddle or the piddle-river always seemed to channel towards their feet. I hated the smell of "pee-toes", so the grass was ripped up as quick as possible and replaced with 3/4-inch gravel.
We currently use 3/4-inch clear/washed limestone. It stays in place, we never have mud, you can hose it down (e.g. diarrhea/vomit), and you can keep it "fresh" by adding salt (some people bleach it).
I've never tried wood chips, but I'd think they would retain the smell of urine. I know when we had grass in the kennel that the soil had a distinct funk to it during the heat of the summer, especially after a rain.
I couldn't imagine life without a gravel kennel and a dog door... oh... and outdoor hot/cold hydrant for our Canadian winters.
Shel