Jason,
Wow, that KM C6500 is ancient. It's all about the media. Does the customer run (or need to run) coated or heavier stocks? The C75XX series may suffice. However, it tops out at 256gsm and crawls when coated stocks are throughput. The C750 offers you superior quality, registration and efficiency (at a higher cost). Also, the C750 offers a more powerful front-end option, additional finishing options etc...