1) Research is going to help your application. 2) Research at the NIH will help your application. 3) Doing research in NIDCR is a big plus and it shows that you are in the dentistry. So why not apply?
It also depends on your situation. If you lack shadowing experiences then doing that for a summer would be better. If you've shadowed plenty then research will make you seem more well-rounded.
I currently work at the NIH and I think this is a fantastic opportunity. You will definitely learn a lot, especially about the other aspects of science like writing, presenting, networking, etc.