The MK3 is supposed to be released next month. They recently moved to a new office, so I imagine they have alot better production line than before. The CR10S is a CR10, but with two Z motors, a filament detect, and a power resume function. The MK3 has many more features, and costs about $150 more than the CR10S. The CR10S has a bigger build area, but I'm sure you can get better prints with faster printing time on the MK3, since the frame is smaller and more sturdy than the CR10S's aluminum extrusions. The MK3 has the dual-tooth bondtech, filament detect, dual Z motors, power resume, a authentic E3D v6 hot end, a amazing PEI coated spring steel bed surface, a temperature calibrated pinda probe, and low noise stepper trinamic drivers. It's really tough to compete with what prusa has designed, as you will have a tough time finding all those features in any printer, let alone modifying a CR10S.

You are probably better off waiting the month on the MK3 preorder, and if you print alot, perhaps.. building a VORON would be more fun for you. CoreXY systems can do large prints easily and won't cost you a small fortune to assemble. However all VORON's are DIY, so you have to buy all the parts and follow the guides on github, and such... YMMV!