I would agree that you should look to increase your range before upgrading your current lens. I shoot with the 17-55 2.8 efs and still often need a flash indoors. I noticed you have many pictures of your dog on flickr. The 70-200 will change your world in getting better candids of your dog. Here's an example from the 70-200 f/4 IS:
Thanks. I do need to get a flash. Does anyone have recommendations of flash for me? I was just gonna grab a used 320 or 270 in the meantime.
I vote for option 3: 70-200mm f/4. Cover the range you NEED before upgrading.
If you got the 70-200 f/4, would you still go after the 100mm f/2? If the answer is no, then the 100mm f/2 is not worth the detour. Getting the 70-200 would also let you reevaluate whether or not you need the 100mm prime.
If you're happy with the 50mm f/1.4 to cover your midrange, why would you consider replacing the 17-40 with the 24-70? Losing 7mm at the wide end is significant unless you don't need that range. If the 17-40 is not fast enough for you (assuming indoor use), then you might want to consider a faster wide prime to supplement your kit at the wide end.
That makes sense. I don't know if I'd pick the 135 over the 100 if I had the 70-200mm. I'd have to see what I needed, but you're right that it's a waste to bother with primes until I have that range covered.
I gave up on the f/2.8 zooms for now, I've had most all of them, and in low light they do not allow fast enough shutter speeds for moving subjects, even with my 5D MK II. They are fine in well lit sporting fields, its theatres or dimly lit buildings that are just too dark.
So I use my 70-200mm f/4 IS outdoors and use fast primes indoors. Since you have 50mm, a 28mm f/1.8 , and a 85mm or 100mm prime might be the next step. You can buy the 85mm f/1.8 and a 28mm f/1.8 for about the price of the 70-200mm f/4L
As soon as you get dancers in almost darkness, it takes a fast prime, high ISO, and slower shutter speed than I'd like to capture a image. Lots of motion blur here.
5D MK II ISO 3200, 135mm L @f/2 and 1/125 sec
Thanks for the suggestion. That's one of the ideas I had been throwing around in my head but was too worried it might sound dumb! I'll definitely try this then. I've ordered the 70-200 f/4L and I'm going to try a lot of things with it when I get here and see if there's an improvement. That picture is beautiful! Another reason I want the 5D MK II. Sadly, that camera is out of my price range for the time being.