Seeing you ride hard enough for an 850, I think its an easy choice. The 600R would be the easy pick for me, because I won't try to keep up on long straights as I don't feel comfortable sustaining high speeds for more than a few seconds.
Funny story, the fastest I have ever gone was 2up on a friend's 2004 600 SDI. *All speeds are on speedo. I had often ridden that sled which was nearly a guarantee to hit 140 kph (87 mph) in any trail conditions, and the odd time 170 kph (105 mph) in the very best condition. This one time I was 2-up to look for missing parts from my Renegade 1000 SDI when I blew the track at 160 kph (100 mph). The little 600 had pulled me home and we returned for the missing pieces. On our way back home my buddy had it pinned on the rail grade. When he saw 200 kph on the speedo (124 mph) he poked me hard on the left shoulder and yelled through his helmet to look at the speedo which kept increasing. When it reached 205 kph (127 mph) he poked me again and yelled "Are you seeing this??? 205!!!" It was still climbing when he had to slow down for a road crossing. When we reached my home I asked what the deal was for poking me so hard? lol He said no one would have ever believed it had happened, so at least he had a witness. Neither of us had ever seen this speed on a sled and would never again with even our higher powered sleds. It was an anomaly.
When Dynamo^Joe made the gearing calculator available on his web site, I remember checking the possible top speed with stock gearing which 196 kph (121 mph) in full overdrive, so the speedo was obviously reading high which was also well known. However, we still had reached 20% faster (205 / 170) then ever before, and 2-up too boot!
The reason for mentioning this literally unbelievable story, is that the 600 HO was a power house which the 600R was built on its success. The problem is what we are asking of it with a 129" or 137" track with 1.25" lugs and a tall suspension. As I witnessed, it has no issues with 500 lbs of rider weight! I this case what it did was squish the whole suspension for the lowest ride and the lowest track attack angle which is what is needed to compete in top speed runs. The other reason was the worn out track with short lugs from thousands of wear miles. I don't know how tall they were new, but I kept the track for historical purpose and they measure 3/4".
If you can spare one more story which follows the last one. On a local club race day I had borrowed the same 600 SDI. In competing with the 800R PTEKs and E-TECs, I would spin out of the whole, but once I had picked-up enough speed I was actually gaining on all of them. I kept trying to adapt the throttle to better my whole shot which worked, but I was still many many lengths behind before I could gain on them. This was in a field with loose snow, so I got the idea to bring the competition to the long 1/2 mile stretch of the trail that went along this field, and challenge these 800s to a race with a running start. I said give me 30 mph before we pin it, and I guarantee the only thing you will see in my back light! Back then there were no 850s, so it was an easy win.
I considered it cheating because I had a 120 Hp 600 that had the upper hand on the setup which was nothing but a worn out short lug track. With a brand new track, and even if the lugs were shaved to 3/4", there is no way the same sled would of out-pulled 800s. Everyone was thinking engine size while I was thinking the ease to turn the track. This goes on to the infamous Arctic Cat F7 stories with their abnormally narrow tracks, and the incredible tops speeds from the Mach Z 1000 SDI having a 1" track, and so on. I have witnessed a brand new aftermarket track that was so difficult to turn that it needed lubricant for the engine to turn on a stand! lol I know, it is another unbelievable story, but I still saw it with my own eyes. It was a Russian made track that was obviously not right.
The point is, a 600 can be made to ride fast enough to reasonably follow the 800s and even 850s, but not with a long track. If I am not mistaken, the shortest track that is offered in a Ski-Doo is 129" with 1.25" lugs. So if I want to follow 850s, I want to avoid the longer tracks like a Renegade 137" or a Backcountry 146". And with a new track, it takes some riding time before it becomes easier to turn. Hopefully what I wrote corroborates with what you already know from experience.
Gearing Calculator
2004 MXZ 600 SDI
22/43 gearing
8200 rpm
1.95 gear ratio
121" track
3/4" lugs (worn down from new)
2.52 Track Pitch
10 Track Driver Teeth
41.8 Constant
196 kph (121 mph) Maximum speed in full overdrive
500 lbs total rider weight (fully dressed)