It is true that for wildlife shots, an APS-C sized sensor will give you a narrower field of view and therefore giving you a slight magnification so you don't need to buy a lens with such a high focal length to achieve the same result. The APS-C sized sensor is also more forgiving, in terms of image quality, if you can't afford to buy the very best lenses with superb optical quality. This is a must with a full frame camera, otherwise you will never be able to fully exploit the advanced features and in so doing get the full benefit.
If your budget is a little tight, it maybe worth your while waiting a few more months as there are growing rumours that Canon are due to replace both the 5D and 7D in the not too distant future and once they do the current models may become cheaper.
I think the next step for you is to try them both and judge one against the other to see which one you are happiest with. You never know, you might just find a clear winner.