I think the storage space is more of a factor than the RAM. From what I've seen in my own usage and read about, 8GB of RAM really is quite a lot, unless you are frequently dealing with processing of very large files. And while an SSD is not as fast as RAM, it is much, much faster than a spinning disk, so the cost of having to swap to virtual memory is not as great as it used to be.
If you are using the machine in multiple locations, external storage is a pain, so you are functionally limited to 256GB in the cheaper model, which is a lot of space, but if you are doing large photoshop/aperture work, will get eaten up. Moreover, even if you have little need for more than 256GB, in 2-3 years when you go to sell it, 256GB is going to look ludicrously tiny, and my expectation is that the resell value delta between the 256/512 versions is going to likely be as great in absolute dollars as the retail price is today--even if 256GB really is a enough, people just aren't going to want to a buy a 256GB non-uprgradeable SSD laptop in a word where 1TB SSDs are common (an assumption, but a safe one, I think).
The same point might be made of the RAM, but SSDs are still relatively immature and capacity is expanding much more rapidly than RAM. 3 years in the SSD market is going to see a lot more change than 3 years in the RAM market.