I picked the 32GB, as I use it mainly for note taking at work, and work mail. For movies and other entertainment, I also have a 12" rMB that is not much bigger / heavier to carry around either.
The only reason I would want to upgrade is, when I decide to use the iPad Pro for photography as well - to take a quick look at the pictures taken from the camera, light retouching etc.
You should think how you want to use the iPad Pro and take it from their, as everyone's use case is different. What I always recommend is to get the largest size you can afford / are willing to spend, as that is usually a leading factor 🙂