I have no experience with CMap so I can't say whether, overall, they are better or worse. What I DO know however is that they include the Explorer Maps of the Bahamas, while Navionics "new and improved" Bahamas maps are still, well, severely lacking (at least as of 2 years ago). If I had depended on the Navionics charts while down there it could have been downright dangerous as the depth lines were frequently inaccurate. I love all of the "features" the companies have built into their cartography, but the first thing I look for is the quality and granularity of the charts. I'm a firm believer that in some locations Navionics will be best, in others CMap will be best, and in others Garmin will be best, etc. for all of the other cartography companies. For US waters there probably aren't a ton of differences as they probably all BASE off the free RNC and ENC's (although I know that they also send out their own survey vessels to get higher granularity), but when you move out of the US there can be big differences.