This is the 850F. The 900F has the X1 Extreme processor, and is compatible with Dolby Vision. The 850F uses the slower X1 processor, and has HLG, and HDR10 only. The 900E uses a completely different stand( I own the 75" 900E myself), and is a 2017. Also, the 900E was not available in 85" size.
While Massdrop prices are excellent, the 900F currently sells for $5000 retail for the 85" version. This price of 3125 is way too cheap for the 900F. MD did the 900F recently, and the 85" was on sale for 4500(retail at the time was 5300).
The 85" 850F sells for $4000, so a price drop of 875 is right in line with MD's typical reduction. The 850F is a great TV. True, the 900F is about 15% brighter, and has Dolby Vision capability, as well as being somewhat sharper contrast, with better color spectrum. I spend a lot of time window shopping at electronics stores.
A store I visit frequently has a display of 850F, 900F, Samsung Nu8000, and Samsung NU7100 sitting in a 2x2 configuration. Directly comparing all 4 TV's playing the same content simultaneously, the 900F looks the best, but not by that much. However, taken on its own, the 850F has a great picture, and if I wasn't looking at all 4 TVs at once, I'd be hard pressed to notice any real world difference. If you don't care about DV, I'd definitely pick up a 850F.
Of course, the best bargain would be the 930E/940E from 2017. It's still rated higher than either the 850F or 900F. It's brighter, and has better contrast, and is also DV capable, using the X1 Extreme chip too. The prices for that have recently dropped. I'm waiting for the 900F 85" version to drop below 3500(which will probably be this time next year, but I'm willing to wait). If I was shopping for a 65 or 75" tho, my pick would be the 930E/940E easily.