I’ve done the half at both marathon weekend (2013) and princess (2015). Both were a lot of fun and both were essentially the same route. There are photo ops all along the race course. The marathon weekend half had more general Disney characters and photo ops whereas the princess one had princess-themed characters and ops. I specifically remember one with Prince Charming “trying” the glass slipper on with runners.
In terms of which one to run, they’re both very similar and IMHO you can’t go wrong with either one as far as the race experience goes. RunDisney does a great job with race preparations, logistics, etc. as far as I’m concerned. For me the fact that the half was not the main focus of marathon weekend kind of diminished it a little bit. It sounds selfish but you essentially get one day to strut around the parks with your medal on before all the marathon and challenge medals take over!
For the princess weekend, the half is the main event. But as mentioned above, the princess weekend is usually the weekend after Presidents’ Day so there are a lot of school vacation folks (like Massachusetts, our home state) and was much busier than marathon weekend.