16GB is probably more important generally, but if you need the storage having only 256GB is tedious. You'll need to know how much your IDE and other programing tools need. I think Xcode needs 20GB or so and games can be pretty big too.
The more portable the Macs get the more storage I want on it, since you don't necessarily need to even bring a charger with you... having to lug around external storage gets annoying. And the lack of ports, then you need to use a dongle to have external storage and charge at the same time. It's just really cumbersome.
8GB is manageable too, if you run up to the limit you can close some apps and such. Or just get used to a few beach balls here and there, with the slower base ssd speed it's probably more important to get 16GB though.