If you are an iPad Pro 12.9" user who bought theirs too long ago to exchange and get the different storage option from Apple, it's best to keep the 128GB. The reason being that trying to sell on 3rd party sites is a chore (and many people are already trying to sell theirs on ebay, craig's list and so on.) So you might be lucky to get around 700-750 (very lucky if you get up to 800) Then there's the issue of we are just about 6 months in to the life of the iPP 12.9" so if you are a person who upgrades each year, spending another 150 or 400 to get the other storage option might not be the best idea (unless you are dying to have it.)
I wasn't planning on moving up to the 256GB... But the thought of trying to sell it & not getting as much as I want then buying another model... Sighs.
When Apple releases the next 12.9 iPP I'll consider going to the 256GB... But I've got plenty of storage already that I think 128GB is the sweet spot similar to 64GB on the iPhone.