I have been shooting with a 1DM3 since they were announced, having upgrade from the 1DM2 and 1DM2N in the past. Prior to that, I shot Nikon, last of which being the D2H. I do a lot of high school sports in poorly lit gyms, and was thinking about upgrading to a 1DM4... in fact I've found one I was talking to the seller about last night. Then I started doing a little research, and now I'm thinking I'll be happier with the 5DM3.
This may seem odd for someone that shoots sports, but bare with me. I am mostly shooting now for pleasure, so its not as important any more that I get a lot of shots of every player in every game. I am taking pictures usually of the same teams over and over, so plenty of opportunities. I hardly ever "pray and spray", and in particular with volleyball its pointless. When I learned that the AF engine in the 5DM3 is the same as the 1DX, that along with the high ISO capability are what pulled me in. I have NEVER been happy with any of the Canon bodies' autofocus compared to the old Nikon D2H, for tracking action shots. The 1DM3 was the best so far for me, but from what I've read of the 1DX, everyone seems to think Canon has caught up with Nikon, and I'd like to experience that. I think to me, the accuracy of tracking outweighs the fps because I've lost a lot more captures due to oof shots, than due to fps. Usually I depend on my timing to get the action, particularly in volleyball which is what I've been shooting mostly.
The other big thing I'm trying to solve is that I almost always shoot with primes, which is a bit of a pain. For volleyball, I'm hauling a 200mm f/1.8, 135 f/2, 85 f/1.8, a few other wides, and a 70-200 f/2.8 on the off chance that its bright enough to use. I'm almost always at f/2, which is a very narrow depth of field, and doesn't allow the offense and defense players to both be in the focus plane. I usually shoot ISO 3200 with the 1DM3 to get enough shutter speed at f/2. I'd like to be able to shoot at f/2.8 and just use my 70-200, and have the luxury of also using the 24-70 sometimes.
I have always loved the 1.3 crop factor. I have only owned one full frame, and that was the original 5D. I got rid of it because I rarely ever used it. I also have a 1.6 crop body, that I also rarely use for sports, but it does the job when I do. I just prefer the 1 series to shoot with... its heavier and more responsive. I have thought long and hard about the full frame vs. 1.3. The extra megapixels offsets it so its roughly the same after crop, and I usually crop everything shooting with primes. However, my shots are often too tight for what I really wanted... no zoom. But I've always been of the opinion that trying to offset with megapixels is not comparable because the larger image in the cropped sensor viewfinder is going to track better than a much smaller target with full frame. I'm on the fence on that one, but the AF engine in the 5DMIII I believe is a lot better than the 1DM4, and definitely way better than my 1DM3.
Decisions, decisions! I can get a used 1DM4 for $3K, or a new 5DM3 for $3K, then have to buy the grip, more batteries, etc.. To me they are kind of a wash I guess.
Has anyone else upgraded from the 1DM3 to a 5DM3 and liked the move? What about vs. going to the 1DM4. What I really want is a similar experience to the 1DM3 feel, but to get a big boost in high ISO and AF performance. Those two things are the most important to me. I'd give up fps and the 1.3 crop. Thoughts?
) since my daughter is on a swim team. So, I need fast glass and the two zoom lenses I have work perfectly for that and for anything else I do. I've always wanted a long lens, wishing for a 400 but I could not justify the huge price jump over the 300, and then I found a deal I could not pass up on a 300. I may add a 1.4x converter and then I'm done with gear, well at least until the next best thing comes along. 
