I have the base model 27" with 512 SSD. No regrets. Money was not an issue; I chose it because it was the coolest-running model. I got it to hold me over until the next Mac Pro arrives, but I have to say I'm quite impressed by it. Stretch for an SSD vs. a spinner and the base model is great. Very snappy, and the fan never spins up much.
The main reason to get the mid-tier model is the faster GPU, but the 570 is already excellent - as fast or faster than the fastest previous generation GPU while running much cooler.
As mentioned, even 256GB is OK for many people since USB3/TB makes external drives fast and flexible, and an external will be much cheaper than Apple's SSD prices.
At present, I have a 1 TB Evo 850 attached via USB-3. It's plenty fast and I don't notice much difference vs. the internal. I'm also looking for a suitable enclosure for a 1TB 960 Evo M.2 drive that I have, which should be as fast as the internal drive. It's nice to just pocket a drive and take it with you.
So yes, you can safely go for the base model. 🙂