There isn't a lot of difference between the two, both are high end companies. Good stuff *is* good stuff. You will be happy with either.
The difference between 850 and 900 is more a matter of how much you want to put into the bag. 900FP down, generally speaking, lofts slightly higher, but is often offset by heavier shell materials and general conditions. Mostly, at that range, it doesn't really matter. 900FP down, in theory, will be a tad bit warmer. But it will also degrade with dirt and moisture a bit faster for the average camper. If you stick to a strict schedule of washing every 10-14 days out, 900FP down will benefit you slightly. If you go out in damp conditions all the time, you won't notice any difference. If you intend to head in to arctic areas(dry), it will make a huge difference.
Between 800 and 900FP down the big difference is the maturity of the feathers when plucked/sorted. Larger plumes, usually means softer, more compressable feathers. But, a large degree of the lofting is suported by the internal electrostatic repulsion between the barbules, too. If you camp under conditions where this can be removed or much reduced, then 900FP is no better than 800FP. We are only talking a small amount of total loft.
Try this with a "lost" down plume(assuming it is clean.) Wave it around in the air while holding the stem. Take a blow dryer on good and warm (just before it actually burns your fingers) and dry it for a minute or so, shaking it as you do this. You can see how it fluffs and seems to expand. Breath very gently on it a few times and shake it. You should notice it got a bit smaller. This is the difference between the electrostatic repulsion and none. Multiplied over the few thousand feathers in your bag, this can have a significant effect on lofting. Wipe it along the side of your nose. This will put body oils on it. Try the same experiment again...
Clean and dry are the two most important things to maintaining high FP down. A down plume can have as much as 1% of its volume flattened and wet compared with clean and dry and fully fluffed.
Keeping the bag dry and clean means extra work while camping, too. Picking a site means picking a dry site. Pick a dry tent, or tarp, with a LOT of vents. Pick a site you can have a fire at. Pick a site that has no sticks and stones to catch and tear at the shell, or, use a ground cloth (usually the manufacturors will use ultra light shell materials for 900FP.) Not an OK site, a *good* site. Even with 850FP down these tricks help, but become more important with 900FP.
High fill down usually found in high end bags means you need to keep the down clean and dry. The higher the FP, the more work you have to put into them. So, this is the trade off between going for 800+ down and 900 down. This is ignoring the dollars, of course. 900FP down is great! But you really need to *keep* it great.