The DR10 is probably the better car out of the box. I do love the Slash because there are tons of upgrades for it, but the DR10 has a lot of drag racing parts available as well. But the DR10 just looks to be better engineered overall.
@Iowa crawler here has built a DR10 with basically all upgrade parts, and it is a pretty wicked drag build. There are some radical Slash builds here too.
So if I were you, I would look around Amain and check out the parts available for each to see what you might like. If you have no desire to upgrade, I would for sure go with the DR10. It is a solid drag car right out of the box. Here is my son Breydon driving his at 9 years old.
Oh, if you click on my Build Thread Contributor badge you will be taken to our drag Slash build. It was built from a standard LCG Slash slider.