Hi,
the only performance difference between the models as far as I’m aware of could be attributed to the RAM size (6 vs 8 GB). But this is likely only an issue with larger or many apps open in parallel. I would not expect the size of the flash (128 vs. 256 GB) to have an effect on anything except the storage capacity.
There is nothing that can be upgraded about RAM or internal flash storage as both are part of the core module. You could expand storage size with microSD cards if that works for you, but beware of the possible issues when taking that route, see for example here: