32gb is fine for me and I still have 6GB left...had my iPad Pro since launch.
See, I don't store movies and lots of music...I stream. I also use a wireless hard drive which has 1TB's worth of movies in HD.
I also thought I needed the space to store stuff in the future, but you know what, I never did save a single movie or a massive file on my iPad. Specially now that we have lots of extra storage choices in the form of external drives, paying premium for a few extra Gb is a total waste of money.
I have an ample amount of apps on it too and I never keep ones that I don't use often. I don't know why people stock apps that they will just use ocassionaly, or will never use at all...it doesn't make sense.
This also pushes me to clean out junk apps and other stuff that will accumulate in time. Having 128gb will make you lazy and in time you will also have to do a clean up due to less space.
Resale value? Just take care of your iPad and get a great resale value at the end. It's all relative...you paid lesser for a lesser capacity and you will get a slightly lesser resale. That's all there is to it.
And yes, I already have a 128gb iPhone 6 Plus which I never exceeded 20gb on.