I've had both, though not at the same time.
**IF** your 852 is set up properly, and nobody messed with The Pots That Must Not Be Touched, an XG is capable of a really excellent picture. If anyone fooled with the pots inside, all bets are off.
The G70 is capable of a sharper picture. Its major advantage is that it's liquid-coupled, meaning you won't see as much light spill from light areas to dark areas of the picture. (On a dark scene with a bright spot -- e.g. a scene of space with the sun in the distance -- you'll see a big halo around the bright spot on a non-LC projector. If you have large bright areas and large dark areas, the halos from the large bright areas will shine on the dark areas, killing the shadow detail in those areas. LC projectors vastly reduce this problem.)
I'd say the LC G70 is capable of a better picture than the non-LC XG, primarily shadow detail. The XG can be harder to set up, but you've apparently taken care of that. The XG is noisier than the G70, and the G70 is a bit easier to quiet if you want to take on the project. I think the XG is a bit more delicate / less reliable than the G70, but finding parts for the G70 is getting very difficult.
The biggest complaint I had about the G70 was the memory model -- which setup memory it chose for a particular signal, and how difficult it was to change manually. It drove me nuts and was a significant reason why I got rid of my G70. But many people have no problem with it so maybe it was just me, or just my particular G70.
Either one will throw an excellent picture, if properly set up. I can't say how much better, but it would be noticeable.