I may be misquoting him but I think Neuro often says the main advantage of crop vs FF is that the crop camera is cheaper. Also cropping 5D3 FF to 7D crop sensor size is very similar except now you have a 8MP file instead of a 18 MP file (for the 7D).
Well, idea is, if you have a bottomless pit of money with full frame you can just buy a longer lens and get better results than crop with a shorter lens. But, the OP has a 100-400 and shoots wildlife; the FOV at 400mm on his current camera is ~640mm. In order to get that on full frame he'd have to spend $10k on a Canon EF 600mm f/4L or start putting on teleconverters which will negatively impact the image quality (and often autofocus) more than a crop camera will.
So yeah crop is cheaper, but its not just the camera but more importantly the lens when you get to longer focal lengths.
As for cropping full frame to APS-C, when you are reach limited there will be more pixels on the target with crop. Cropping full frame will work but you will lose detail because of this. The exception to this rule would be when shooting in low light (i.e. ISO6400) when the lower noise & higher contrast of full frame would likely be a better tradeoff for a little less detail. Some feel that cropping full frame is at worst no difference but that is not my experience at lower ISOs and it does not logically make sense to me knowing how digital imaging works.
jrista did a fairly well controlled test here that can demonstrate the loss of detail when cropping full frame vs using a crop camera on a reach limited target for a visual demonstration; you can see in his test the full frame crop loses a ton of detail on the moon surface/craters because there just aren't enough pixels to fully reproduce it on full frame due to the cropping: