500 f4 is of course a gorgeous lens, but it is too large and too expensive for most of us mere mortals. It is also a bit overkill for zoos (works great with wild birds, however). I think your idea of the 70-300L is a great choice for zoo photography. It gets very good reviews. Just remember it does NOT take extenders, but 300 should be enough reach for zoo work. One alternative, the route I went, is to get a 70-200 f2_8 and use it with a 1.4x extender when needed. I bought the non IS version last year, as it is a thousand bucks cheaper than the ISii version. They have now discontinued the non IS, but some dealers still have stock.