Canon 5D or Canon 1D Mark III ?

For the work you do, between these two options, I would go for the 5D Mk II without hesitation. You can use the smaller RAW format if file size is an issue, or shoot at full res. if and when you need it. For your indoor work you get the advantage of full frame with full wide angle coverage - you will of course need the best lenses you can afford. If your work was more sports and journalism oriented, in inclement conditions, the 1D MkIII would be a better choice, but if your 20D has survived without problems you would be paying for features you probably don't need.

The whole "Pro" thing always seems a bit of a misnomer to me - the question is really whether or not you need a more rugged, more water resistant body (none of them are "waterproof"). An amateur who shoots on yachts or mountaineering needs a 1D series body much more than a professional who shoots fashion or studio product - the "Pro" designation is marketing nonsense really - "Heavy Duty" would be a better description.

Obviously the final decision is yours and the truth is that both are excellent tools and neither one would be a "bad" choice. If you like the advantages of both and want a to combine them, the 1Ds MkIII has it all (except the video, which probably is a non issue)!

If you are not interested in speed then the the 5D2 is for you. The 5D2 also has an LCD screen showing you the f-stop, ISO etc without looking through the viewfinder. That is not just a feature exclusive to the 1D3.

Yes, the 1D3 has file sizes that are more manageable but the 5D2 has sRAW1 and sRAW2 which can give the same size files as the 1D3 if required.

Although the 1D3 has more AF points and has faster AF I would not say it was more accurate. Remember that the 1.3 crop factor also has its disadvantages. It is an awkward camera for true wide angle photos. The 17-40 f4L acts as a 22-52mm lens and you can't use EF-S lenses. The only zoom lens that will give you a really wide angle on the 1D3 is Sigma's 12-24mm.

If you like shallow depth of field for portraits the 5D2 will make it easier to obtain it with its full frame sensor. The 5D2 is also better at high ISO than the 1D3 (albeit slightly).

On the downside, the 5D2 has poor build quality in areas (battery door, CF card door). The joystick control is also awkward to use. The AF points are all clustered in the centre of the frame making some off-centre subjects awkward.

I had the same decision to make and I chose the 5D2. Contrary to popular belief, the 5D2 is a quick camera. People use its frame rate of 3.9fps to label it as a slow camera. It is not. It has the new Digic4 processor and it operates quickly. It is not all about frame rate. Its shutter response and the speed it writes those huge files to the CF card is quite incredible. I also don't like cameras with a built in grip which is another reason I avoided the 1D3. As I said earlier, if you don't need outright speed and the build quality/weatherproofing of the 1 series, the 5D2 should be your best option.

I do landscapes, nature, animals, portraits, events, and indoor photography like for restaurants and hotels...
Landscapes, portrait and indoor photography are asking for full frame, most animals can be done with full frame, with the exception of birds, where a crop camera would be better, looks like the 5d2 fits the bill or if you want the faster camera also you could go to the 1Ds3
Ross

Pro level = 1D series full stop.
Before all the 5DMk2 users bite back: I'm not trying to rattle your cage.
The build quality and the interface are what determine the pro quality here.

All that said. I think the 5D Mk2 better fits the demands of the poster. But it ain't pro level.
Unless you really need 10fps or hyper blistering (rather than just blistering) AF then there is little else to recommend the 1D Mk3 over the 5Dmk2

The is great post so well and articulately posted, I am in the same dillemma. I shoot with a 500 f4 L IS + 30 and looking for a body UG, still abit confused.

Hope some experts will give good advice here.

LOL - So... I own and shoot with both 1Ds MkII and IIIs and 5Ds. I have been a full time professional for 25 years, also shoot 4x5 Sinar, Mamiya RZs (scan on an Imacon Flextight) and Hasselblad MF Digital. My 4x5 and MF cameras are not heavy duty or water resistant and therefore presumably not "pro level". In fact I guess that the only time I am using a "pro" camera is when using a 1D series Canon.

Since when did the water resistant abilities of a camera have anything to do with whether or not you use it to earn your living? Clearly the "Pro" thing is a marketing, ego driven designation and it just muddies the waters when helping people decide what tool they really need.

The Mk 3 also has two storage slots which usually are set up for raw on the CF and jpeg on the SD card. Never have to wory about loosing images. The battery charge just lasts and lasts, if that is important to you. I like the speed of the auto focus. Do a search for the white paper on the Mk3. There is probably one available for the 5D Mk2 also.
I have the 5D Mk1 and the 1DMk3. I am always shooting with the Mk3, I like the feel of it although it is heavier than the 5D w/o the grip.

Your probably little surprised at the responses, but I have to agree with most that with the type of photography you are going to be using it for, the 5DII is probably your best choice. They are plenty rugged enough for what you will use it for, think about what you have been using and it's lot better built then a 20D. Also the extra WA coverage for indoor shoots like you mentioned will be big help. You also mentioned wanting the extra features and 3" LCD, well the 5DII has them all, 1DIII doesn't. Wait till you see the LCD on 5DII with high resolution screen, it will make your 20D screen look pretty bad.

Either camera can take "pro level" photographs. The real question is not "which one is more pro," but which one is best suited to your particular photographic needs and tastes.