I am going to stick to the 85gr. or the 100 gr. at the present time just due to the availabililty at any gun store. What grain would be the most versatile?
I think the 100 grain would be most versatile. The reason is shooting at deer at 400. I don't have my ballistic software right here, but I'm inclined to think that 400 would be at the outside limit of the 100grain bullet in the 243 when shooting deer.
Shooting at yotes is a different story. With the 100grain pills, you could shoot well past 400 yards on yotes.
what would you sight it in for?
You have couple of routes to go here:
1. Sight-in for 100 yards, then dial your scope for the range of your intended target.
2. Sight-in for point blank range. That is to say, a deer has an 8" kill zone. You would need to find the distance where your bullet neither rises nor falls more than 4". That way, if a deer steps inside that range, you know you can hold dead-on and hit him in the vitals. Beyond the point blank range you would have to hold over or dial for the distance. My guess is that your sight-in distance would be around 250 yards.
3. Knowing that your point blank range is probably in the 250 yard range, I would just sight-in for 200 then dial the distance of any longer shots. I would go this route because you will probably find that your point blank range is some odd number like 238 or 263 and most drop charts are in increments of 25 or 50 yards. With a 200 yard zero, it makes it easy to dial the distance based on your drop chart.
possiblilty of using BOTH bullets and just switch them out for whatever game I am after?
This is all about knowing the trajectory of any given load in your rifle. You can easily have both cartridges sitting next to your rifle and drop in the one you want for the game you're going to shoot. Then refer to the appropriate drop chart for making the shot.
My recommendation would be to simply find the 100grain load that shoots well in your rifle and go with that. Then get to know that load inside and out. So regardless of whether it's a deer or a yote, you are confident in placing the shot.
Have fun.