Ten people, ten opinions. Completely pointless question - it all depends on how many games you want to have available at one time. Every game is different in size, 256gb can hold 200 Indie games, 15 space-demanding AAA titles or any combination in between.
How easy upgrading is depends on how you set up your console. If you hack your Sysnand, it's just a matter of copying everything from SD1 to SD2. If you set up an Emunand, you'd have to copy that partition as well. But that is another topic.
I'd say spend the extra 10 bucks and get 400GB. That will hold tons of games.