The 6D has a larger sensor than the 7D, that's all it means. So when considering the image that a lens projects onto the sensor, the 7D (and all other DSLRs with a APS-C sized sensor) "crops" the image. You still get everything you see through the viewfinder in both cameras.
In my experience, the 6D has the advantage over the 7D. I shoot more for print, and therefore noise is a huge factor. Theoretically, the 6D has lower noise in higher ISO situations, and this is generally true of full frame cameras because their sesor pixels are less dense. In the real world, it depends on the situation but I've had better results with the 6D (which has exactly the same sensor as the 5Diii).
Also I prefer full frame because I've found shooting wide angle on an APS-C sensor is very challenging since you run into distortion problems when accomodating the equivalent angle of view (10mm lens vs 16mm on the full frame, 14mm vs 24mm, etc). You don't have that same problem when shooting telephoto, and the "telephoto advantage" is really a myth, since you would get the same results "cropping" a full frame image (in this case "crop sensor" is appropriate).
I honestly see no reason not to go full frame. It's better all around, and trust me, the 6D is very well bulit. The ONLY disadvantage it has compared to the 5Diii and 7D is fewer focus zones (not a big deal), slower shooting rate and video features. If you'll be shooting a lot of pro-level video I wouldn't recommend the 6D.
