It depends on your needs. 300 is nice to have for weddings, but most of your shooting isn't going to be from that distance. You also should note that the minimum focus distance on the 300 is 2 meters. If you were to get the 70-200 and use a teleconverter you could get to either 280mm at f/4 or 400mm at f/5.6 with a minimum distance of only 1.2 meters which is substantially closer.
There is also the fact that the 300mm costs more than 3 times that of the 70-200mm. You could also get a 100-400 f/4-5.6L and a 70-200 f/2.8 and still have a significant savings left over which is slightly better at 400mm than the 70-200 with the 2x tele.
While the 300mm f/2.8 is a great lens, if you are doing weddings, I just don't see how it is going to be useful to you. Being a prime it will be limiting and it's really too far to have to shoot from. Also, at 5.2 pounds for the lens alone, it isn't going to be able to be used by hand. You'd need stabilization which also wouldn't be wedding friendly. It's really more targeted at sports and wildlife than events.
Disclaimer: I personally just purchased the 70-200 f/2.8L IS II to compliment my 24-70 f/2.8L II and am contemplating the same teleconverter suggestions I made to you. I've actually got trial versions of the extenders coming in a couple weeks. I've been very pleased with the lens, it is super fast and remarkably sharp, though not quite as sharp as the 24-70 in my opinion.