Headbutting the chest, even with a kevlar helmet on could be risky as you are basically hitting center mass with your head (at least if we're talking about a standing situation, if in side control, guard, or north/south it would be safer). If the opponent is bigger than you and/or just happens to aggressively close the distance as you are throwing the headbutt, that is a lot of force that your neck muscles must be able to hold up against, especially if you don't catch the chest at the optimal point in the force curve.
If you were going to do it I'd probably aim for the sternum, or slightly off to the sides, attempting to affect the heart/lungs and/or possibly dislocate a rib or two. Of course, it would also depend on alignment to one's opponent. If off to the sides, the actual heart or possibly the liver (depending on height of the opponent) could also be targeted.
Headramming the chest on the other hand could be effective, especially if you combine it with pulling on the backs of the knees (basically a "blast double/high double" from wrestling). Very hard takedown/landing and doesn't mandate that you go down with your opponent. Again though, you're got to have your bone structure aligned properly, or you could wind up really hurting your neck, or even getting paralyzed.