I bought based upon my storage needs so I went with the 512GB model. I was using about 180GB of the 256GB on my last model, so now I have room to spare.
I figured that if 8GB is good enough for default configuration of the MacBook Air or new iMac, than it’s more than good enough for my iPad usage. I don’t edit videos or do complex 3D modeling and 8GB will be more than enough for doodling.