Depends on what you're planning to do with it and how long you want to keep it.
You already have an iPad? Then look hie much storage you have left and then decide of you need/want more. At least that's how I'll do it, when I replace my 1st gen 12,9" iPad Pro.
I would get 512gb I am down to 265gb on mine already but I carry a lot of. Intent with me on my travels. Better to have it and not need it than have to work with workarounds.