They're incremental upgrades, primarily adding 3D to existing models.
Samsung is one of the companies that's really pushing the 3D craze, but until recently they've only had a single 3D capable Blu-ray player to offer. The new releases expand on that and make that lone player feel a little less lonely.
The $230 BD-C5900 is essentially a 3D capable BD-C5500. It's got the same specs and the same look, but with a big 3D logo planted on the top. The $280 BD-C6800 is the same, only it's an upgraded 6500. Both players sport streaming features, upscaling, Blu-ray 3D compatibility and HDMI 1.4 outputs.
If you're looking for a pair of HDMI outputs, rather than the single port offered by most players, the BD-7900 has just what you're looking for. It's an upgrade to the C6900 and comes in at $400.