It sounds like a fun activity, I'd suggest going for the 50. Sure you already know you can handle that distance, but as you said, this is a race and you are going to want to push yourself to go faster. As a first timer to the event, it would be an achievable distance and at the same time not over-before-you-know-it short like the 25 would be.
If after you are done you feel like you could have done more, then you know what to do for next year/next race.