Dirt is more fun to drive on imo, but you much more car maintainance then on astro. When astro is dry, you have just too much grip with the right tires and we cut the outer rows of pins of the tires. The tires wear very good on astro. Actually, a tire with some wear that is broken in, performs better than new tires on astro.
I race on both surfaces and I enjoy them both. When I race on astro and I come home and I put the car on the self until the next race. I might change the diff oils and shockoils, but that's it.
When I race on dirt and I come, I start tearing the car down to get all the dust and sand out of it and clean all the hinge pins for a free moving suspension.
The only thing I hate about astro and dirt is that the setup is totally different so when I change from surface. On astro I take as much grip as possibly away and on dirt you want the opposite ofcourse.