You're double-counting the discount. If you're getting an iPhone 6 either way, the -$600 you have above doesn't apply. The very link you posted explains it best. You should be roughly indifferent between Next and up-front purchase. Both of which are slightly better than on-contract without Next.
ETA: They're doing this because T-Mobile's off-contract pricing is smokin'. Only way they can compete.
that's what I figured was happening, but that link is confusing then because under the NEXT column it shows this for monthly:
Access charge: $40
Family share discount: -$25
Phone financing: $37
Total monthly: $52
Based on that, the family share discount still gets applied. That $37 lasts for 20 months ($740) and you pay $15 for 24 months ($300) to get $1040 over 24 months. I'm not sure where he got $1109 from but it's not a huge difference.
Going with the 2-year contract plan, you'd pay $299 up front and the $40 upgrade fee. Then you still pay $40/month without the discount. So, it's $340 plus $960, which gets you to $1300 total cost.
So, in this scenario he posted, Next saves $200 over the contract plan. But, I guess you're still paying $740 for the phone over 2 years, which means it's not subsidized.
Neither option looks great but Next certainly looks a lot better than the stupid contract.
Either way, guess I'm not upgrading. Not interested in a $700+ phone and like my current setup.