Other than the jump in pixel count I don't think there's any major improvement between the two. What do you shoot? & are you having to crop a lot? It's not a cheap upgrade, and from a lot that I've read many go from the 1D3 straight to the 5D3. Would you be able to live with fewer FPS while enjoying a better AF system & higher ISO performance? How high of an ISO do you consider acceptable on your 1D3?
I was under the impression the 1D3 could do higher ISO than that. I know 1600 is a bit noisy on my 1D2n & 1Ds2 but if the exposure is on the money they are usable. I do have a 1D4 which I bought in the spring & have used at 3200 & 6400 with decent results but I haven't really compared those results to what people are getting with the 5D3. From everything I've read the 5D3 AF is better & so is the higher ISO's. I spent a lot of time deciding which way to go & I'm happy with my 1D4 for my specific use which is primarily to shoot radio control events (good light) & was more interested in having more pixels per duck & the 1D4 edges out the 5D3. I will be finding out just how well it works in low light next week while on vacation. Carnaval (Mardi Gras) begins the day I arrive for my vacation & I'll be shooting the nightly parades & some of the other nightly stuff. I'll be trying both ISO 3200 & 6400 plus a flah.
The 1D Mk IV has several upgrades over the Mk III. One being the better LCD screen and menus. Much better, yes, way much better.
45 point cross point focusing compared to 19. 16 million vs 10 million sensor. Four levels of NR instead of two. Higher ISO.
I've been considering the same upgrade. But I'd have to lose both a 1DIII and a 5DII to make it.
Advantages of the Mark IV are Digic 4 processing, contrast detect autofocusing in live view, video (if you use it), more resolution, improved phase detect AF and high ISO compared to the Mark III.
The big improvemnt for me would be resolution for sports and wildlife but I'd be losing resolution for landscapes. Most recently, I've been thinking not to make the changes.
Interesting thread. I sold both a 1DM3 and 1DM4 in order to upgrade to the 1DX. I didn't get a whole lot for the M3 due to the focusing problems of that model. Mine had every "fix" offered by Canon applied to it including sending it back for one of them. The M4 is a much better camera although it would occasionally "freak out" during a photo session and run the lens focus to one end and refuse to focus until you let up on the shutter release (or AF On button if using it) and tried again. It didn't happen that often but, when it did, it was aggravating. I haven't really experienced any focusing issues with the 1DX.
As for your question about the upgrade, even if you aren't experiencing any focusing problems with the M3, the M4 will give you better low light images plus the extra 6MP for cropping. I don't know if you've already researched pricing for the upgrade or not but, when I sold mine a few months ago, the M4 brought a pretty good $ amount because quite a few wildlife photographers were still hacked off about the 1D X full frame sensor meaning they would be losing the 1.3x multiplier for their lenses...
I'm back from my vacation & did get to try some real world high ISO shots with the 1D4 and I'm impressed. The first 2 were shot hand held from about 100 yards away using the 28-300 L IS at 300 mm. (these are as shot jpgs, no processing done, resolution lowered to post here.)
These are with the same lens but from much closer.