I've shot my own dog while on walkabout with 17-55, 50/1.4, 85/1.8, 100L, 135L, 70-200/2.8L IS, 100-400, 120-300/2.8 OS. They all do great so I wouldn't sweat the decision too much. I would say that longer focal lengths normally make the shooting easier, as you don't need the dog to be on top of you and as distance changes in absolute terms (metres/feet) it changes less in relative terms (percentages). Actually, looking at my originals I would say that cropping has been the order of the day with the shorter focal lengths, so I'd recommend going longer than shorter.
Here are a few examples....
7D and 100L, cropped :
| IMAGE IS A REDIRECT OR MISSING! HTTP response: 404 | MIME changed to 'image/png' | Byte size: ZERO |
1D3 and 100L, cropped :
| IMAGE IS A REDIRECT OR MISSING! HTTP response: 404 | MIME changed to 'image/png' | Byte size: ZERO |
7D and 85/1.8, cropped :
| IMAGE IS A REDIRECT OR MISSING! HTTP response: 404 | MIME changed to 'image/png' | Byte size: ZERO |
1D3 and 24-70 at 70mm, uncropped :
| IMAGE IS A REDIRECT OR MISSING! HTTP response: 404 | MIME changed to 'image/png' | Byte size: ZERO |
7D and 135L, cropped :
| IMAGE IS A REDIRECT OR MISSING! HTTP response: 404 | MIME changed to 'image/png' | Byte size: ZERO |
7D and 50/1.4, cropped :
| IMAGE IS A REDIRECT OR MISSING! HTTP response: 404 | MIME changed to 'image/png' | Byte size: ZERO |
I do tend to agree that managing with the 70-200 is probably the best, and cheapest, overall solution. I have tired somewhat of shooting my own dog, but when I'm in the mood I'm happy to lug my 120-300/2.8 OS about for a serious bit of dog shooting, and that makes the 70-200/2.8L IS seem like a toy by comparison, in both size and weight. A Black Rapid strap is the solution that works for me.