Head coaching experience will definitely get another small college athletic director to look at your resume, but if you want to be ultimately be in DI, take the DI job.
DI athletic directors, or those that come from DI and are at smaller schools are snobs, they only want DI candidates.
If you can see yourself staying at this NAIA school and building a program long term, it can be very rewarding, and set up a good career in small college athletics. But be prepared to take on additional tasks in the athletic department to be able to build a career. (many small college full-time coaches, that don't do administration, can't make it salary wise once family enters the picture).
If you are looking for a stepping stone, take the assistant position at DI, if you are looking for a long term commitment take the HC position.