IMHO If you need space, go for space! If you need RAM, go for RAM.
8GB RAM is a minimum on todays setup, but if you don't use any memory hungry software anyways, it's a waste to upgrade to 16GB.
Also, 256GB data is a lot, you can transfer data to an external drive, NAS +++, you'll always have room for data.
You cannot upgrade the RAM - EVER, but you can expand the storage through external devices / cloud etc.
In any case, go for what is needed now and throughout the computers life-span (7-8 years). But if you're not going to run a couple of virtual computers on your Mac, the 8GB is PLENTY!