I think you should do the Disney 1/2 marathon (not the Princess). I'm sure the Princess will be great, but for your first I think you will like the grand scale of the January 1/2.
I have done the Disney 1/2 in January and I did the "Minnie Marathon 15k" last year, which is replaced this year by the inaugural Princess 1/2. The Minnie Marathon (now Princess 1/2) is significantly smaller and does not have the same crowd support along the way. In January, you have all the people who are there for the marathon, and the cheering crowds are so much bigger. Plus, the entire resort is geared up for the marathon weekend in January, while the smaller race in March was less of a big deal resort-wide. Also, as the March race is marketed mainly toward women, it's a much different atmosphere.
I wore my medal around the parks after the big January race, and got many "congratulations" from cast members and guests alike. I LOVE that!! After the March race, however, I wore the medal around and no one really noticed... most people didn't even know there was a race that weekend. I recognize that this is a minor thing, but it is worth noting!!
I think the January race is SIGNIFICANTLY better if you want a big-race, big-motivation, big-thrill, resort-wide all-out-Disney-running experience. The March race is better if you want a smaller situation with less hoopla. I personally prefer the January race by a long shot. Best of luck, either way you'll have a great time!!
