But more often, you'll need all the length you can get. I find that even with my 500 f4 IS USM and 7D at 100 yds (the minimum distance the rangers let you get close to anything interesting) I'm still cropping. With your FF, that will even be worse. So, if you're really serious about wildlife, you might consider renting a 600mm. On the other hand, lugging the big whites around is a pain, and you'll need a tripod (and big backpack, if you're going off the road...). If its more about recording your vacation, I'd say stick with the 70=200 and the 2x.