I am both a Canon and a Sigma X3F (Foveon) user, and there are good arguments for both sensors. There is no question that the Sigma CAMERAS are deficient in many areas, some not related to the sensor demands, but that for specialized uses, mostly landscapes, the Sigma Foveon cameras have unique qualities making it worthwhile to put up with annoyances. Canon cameras are good all-around cameras, Sigma cameras are specialty cameras.
The Sigma Foveon sensors are perceptually sharper, per pixel, than the Canon Bayer sensors, 15 Mp APS-C DP#M sensor is sharper (slightly) than my FF Canon 6D 20 Mp Bayer sensor, and users of both Sigma and Sony/Nikon FF 24 Mp sensor rank them as similar, with the Sony sensor minimally better in sharpness.
The major rendition difference in the Foveon and Bayer sensors in the current iterations is that there is a certain color subtlety in the Foveon sensor RAW files, sometimes called "film-like", that is not present in the Bayer sensor RAW files. Low local contrast, hue-restricted areas are considerably more detailed on the Foveon sensor files than the Bayer sensor files.
To my mind, the combination of color subtlety and acutance is the one and only reason to go for the Foveon sensor. Foveon sensors excel at landscapes and floral portraits, and are "too sharp" for most portrait use - one is likely to need to do more blemish-removal post-processing than for Bayer sensor files.
Canon Bayer sensors: very well developed computational protocols mean fast high-throughput processing. Great for action because you can get very high still frame rates and you can pack more frames into the buffer.
Sigma Foveon sensors: fewer generations of computational protocols, new ones being tested, current (non-Quattro) generation is s-l-o-w, still frame rate is maybe 3 fps, and it takes several seconds to clear a full (7 photos at ~55 Mb each for the 15 Mp DP#M/ SD1M sensors) buffer.
Canon Bayer sensors: nearly "infinite" post-processing software options that work with RAW files. Seamless integration of your RAW developing program with external programs and plug-ins.
Foveon sensors: 2 RAW processors, Sigma Photo Pro and Iridient Developer. If I want to make a panorama, I need to do the RAW color / contrast / exposure adjustments in Sigma Photo Pro, then export as a 16 bit tif into my pano program. Ditto for HDR program. SPP gives good results but lacks some exceedingly simple local editing maneuvers such as cropping (!). On the other hand, SPP monochrome mode makes gorgeous B&W images from your color-adjusted (fake filter) RAW files.
I have two parallel workflows, and this is a PITA. I have two parallel file trees, one for Bayer, one for Foveon files. I use Lightroom as my organizer and RAW developer, and LR does not recognize (Adobe likely NEVER will recognize) the X3F Foveon files. So, if I want to catalog my Sigma images, I have to export a small jpg next to its parent file, and acquire that proxy jpg in LR. Then, I can score, tag, keyword, etc., but in order to work with the X3F RAW file, I have to leave LR and manually go to the physical location of the X3F file, launch SPP, etc.
I will be lining up for the Canon 7D2 for bird photography (replacing 60D), and will be very interested in the Canon Foveon-like FF sensor.