Toby is right, this is a personal decision. The large road races are generally flatter, and will have lots of support like more water, medical and the ability to probably bail on the run if you get hurt or can't finish. On the trails, you will have less people, probably better secrenary, but less water stops (take your own), less porta-potties (use the local tree) and less medical care and you will probably have to finish as there is no taxi along the way to pick you up. I have done both several times over and like each type for its uniqueness. Enjoy your first marathon. you will love it.