The 5DMKIII does have more MP but has less density the chip is 1.6X bigger therefore pixels are more spread out. APC crams 18MP on a sensor that is 1.6X smaller therefore the 18MP APC chip has the largest pixel density of any Canon sensor meaning more pixels on target and a sharper image. That image sharpness depends on the ISO, 1600ISO is about as high as you would want to go. The density makes the sensor pretty noisy, which is the benefit of full frame.
Also if you use a 1.4 with the 70-300mm you will get a slower focusing high aperture lens so you will have to compensate with ISO on the 5D anyway. the 70-300mm with a 1.4 will be a 98-420mm F5.6-8 lens, which is pretty slow and the AF will be even slower.
With a 7D you get 112-480mm F4-5.6 lens with full speed AF and you could still put a 1.4 on it if you wanted making a 157-672mm F5.6-8 you also get 8fps rather than 6fps, and more depth of field on the subject.
An image cropped from a 5DMKIII and upscaled to 7D size will perform similarly. But the 5DMKIII needs more sharpening.
The fact is if your shooting in good light crop is better for wildlife if you want smaller, lighter and more length for your $ if you don't have the funds/don't want to carry the big whites.http://www.dpreview.com/forums/thread/2791104
Why not take the 16-35mm and the 24-105mm, the main reason for the 24-70mm F2.8 is the F2.8 will you need it? also has no IS, much heavier and much bigger (if your talking MKI). The 24-105 fills the gap has a better range and IMO is the best walk around lens maybe not the sharpest but its a very good lens. I would have two bodies with the tele you choose and the 24-105mm attached to the other.