The Bluecharts on the Colorado 400c are built into the firmware of the unit. No CD or SD card required. On the upside, that does leave the SD card slot vacant for adding additional software, or just expanding the storage.
In that case, the Colorado is about $A100 more than the 76csx, but you save about $200 on Bluechart. That means you are paying an extra $100 to be able to load Bluechart on the PC. It's a value judgement, but I would still go with the option to use Bluechart on the PC. It's great being able to view a detailed nautical chart on a full-sized screen. I've already found that valuable in one way: There's a bay I would like to be able to launch from, but it is basically a mangrove swamp, with a maze of intertidal islands. With the Bluechart, I was able to see that there is an unmarked channel. I was able to plot a route on the PC, then download it onto the GPS, so hopefully that will enable me to negotiate the maze.
Also, if you upload a track log from a trip you have done, the track displays on the map. You can do that with Google Earth, but with the Bluechart, if you want to analyze the individual legs of the journey, each leg is highlighted on the map while you are looking at its stats. That would have been very helpful for Titan on the general kayaking forum, when he was locating his lost Mirage drive.
Also, you can put as big a microSD card as you wish in the SD slot, if you want to load additional software.
Disclaimer: I've had Bluechart a total of 3 days, so I am hardly an expert.