From what it looks like so far, the i9 doesn't have a HUGE benefit compared to the 2.2 or 2.6 i7. It is certainly the fastest CPU of the bunch generally speaking, but value wise I think it depends on what you do.
Personally, I wanted the 560x and at least 512GB storage - so by the time I upgraded the base 2.2 model with those items, the 2.6 i7 was only $80 more - which was a no brainer to me. So I went with the middle of the road. 2.6/512/16gb/560x.
For what it's worth, I tested the base model 2.2/555x at the store and it was still absolutely blazing fast. I don't think you can lose with any of them... but the best value is the middle stock model 2.6/512/560x/16gb if you plan on upgrading those items from the base model anyways.
I really wanted 32gb ram and 1tb drive, but apples upgrades are insanely expensive. It's more cost effective to buy what you need now and resell and upgrade in 3-5 years. At least that's my plan.