The C6100 is built on a completely different platform than the C1060/3070 series. It is much more robust, the fuser and paper path are significantly bigger, and they way they handle registration is different. The C3070 physically moves each sheet into the same position with a swing roller, while the C6100 platform uses extremely fast measurements to move the image onto the paper into the same position every time. We find the registration of the C6100 to be excellent. The C7100, however, is not a true upgrade to the C6100 as I've indicated in my other posts. Unfortunately, KM went backwards and built the C7100 on the C3070 platform and just sped it up. This has resulted in a machine that can't hold color consistently among numerous other issues. The C6100, however, was and is a solid production machine. We have one in our shop (actually the C6085 which is the same machine, just 85ppm instead of 100ppm).
First, I would ask: Why are they trying to make this switch? Did you ask for an upgrade or are they pushing this on you, and if so, why? Are you seeing a significant increase in volume? Or are they just trying to make a sale?
Second, be aware that the C6100 is no longer sold as new and hasn't been for a couple of years now, so they are going to give you a used machine.
Third, you should know that the color and toner sheen between the C3070 and C6100 is quite different. The color on the C6100 is not better or worse, just different. So if you have some color-critical jobs/customers, then they will notice a difference. The toner in the C3070 also tends to be a bit more shiny when compared to the C6100, which can sometimes result in the colors looking more saturated on the C3070. When I used to sell production gear for KM, I had a demo room with a C3070 and C6100 which were both regularly calibrated. Customers would run the same job on both machines, and they tended to like the output of the C3070 better. However, if they didn't hold them side-by-side, the output of the C6100 was just fine on it's own.
Fourth, if you run envelopes, switching the fuser on the C6100 is much more of a hassle than the C3070. You actually need two large metal carts that are designed to load/unload the fusers. These carts are pricey and take up alot of floor space because the fuser is so much bigger/heavier.
The main benefit to switching to the C6100 is going to be the speed/productivity. Not only is the speed faster in general, but it maintains full-rated speed on cardstock where the C3070 will slow down a bit. The color consistency and registration is about the same between the two.
