A lot of people think that with the crop vs FF mag factor, but sadly, its only a smaller FOV. The sensor can't change the magnification of a lens. However, as always, thats not the whole story....a decent crop sensor has smaller pixels, packed more closely together than a FF sensor. So that means its a little less light sensitive ...but... normally with wildlife photography, you need to crop a lot of the image away as the bird or whatever, is quiet small compared to the rest of the frame. The bit left after cropping should have more pixels (data, detail whatever) than the equiv full frame sensor. It also means you don't need to carry a heavy FF camera around, which is a saving in itself! Thats assuming a decent crop vs a FF. The most high end FF available have a very high resolution and are very good still when cropped down.
On the IS, thats really a good one to test in the back garden against any local wildlife or cats. Only you will know what works for you. Monos are great when you need to carry them, thats very true!
