I have an entire house to tile, am about 2/3 done with the living/dining room, my MK-470 wanders when doing a long cut, the new saw will make a lot better cuts. I'll sell the MK-851 on Ebay when I'm done with it...
I used an old fashoned
tile cutter when I did my last house, the tub saws are a lot nicer to use, no sharp edges, fewer wasted tiles, and I've got a thing for power tools