The 5DIII has dual card slots, one for CF and one for SD. I've never had a dual slot camera, only one slot cameras, either CF or SD.
So I need to figure out what sort of CF and SD cards to get, and if I'll get both. I've always perceived CF cards to be 'better', both in reliability and in speed, but I'm not sure that is true presently. SD cards do appear to be cheaper, even in the high speed incarnations.
I am looking at 'fast' cards, since I want to be able to use the 5DIII's 6 fps shooting capability both for RAW and jpgs. Supposedly the 5DIII can shoot 6 fps jpgs with a fast card (only CF or also fast SD?) until the card is full. This suggests that it can shoot maybe at 3 fps RAW until the card fills up. Maybe CD and SD cards are now equally fast.
In any case, the camera specs (the user manual isn't out yet), say
* Records to either the CF card or SD card.
2. Auto switch card
* When the current card becomes full, the camera switches to the other card automatically.
3. Record separately
* The CF card and SD card record the same image at a different image recording quality (L, M, S1, S2, S3, RAW, M-RAW, S-RAW)
4. Record to multiple
* Both the CF card and SD card record the same image at the same image recording quality. (Also applies to RAW+JPEG, M+JPEG, and SRAW+JPEG)
Images recorded in a card can be copied to the other card
Given the costs for fast CF cards, I'm looking at a 32 GB CF card and either a 32 or 64 GB SD card. If multiple recording is as fast as single recording, I'll probably go for that with two cards, with the 64 GB SD as a backup if I run through the 32 GB on the CF card. The logic here is that I currently have a 8 GB and a 4 GB CF for my 40D. RAW file size for the 5DIII is maybe (see below) 2.5 times bigger than for the 40D, so I need more that a 16 GB card to be comfortable...I'm quite able to fill up a 8 GB card in an afternoon and I do run out of card space on a good day with only 12 GB of CF card space. 32 GB CF should enable me to do what I want, with the 32 or 64 GB SD as a backup/overflow card.
RAW file size on the 40D is in the 12-15 MB range, on the 5DIII RAW file size appears to be in the 25-40 MB range (there is lossless compression, so file size depends on the image). I'm looking at the Lexar Professional 1000X CF card, one of the first UDMA 7 cards, which on the Canon 1D IV obtains write speeds of 65 MB/s. Speeds on the 5DIII should be better, since I assume the 5DIII is optimized for UDMA 7 cards, but the 1D IV is not (need to check this...it seems the 1D IV does 'support' UDMA 7, but not in a manner that optimizes speed, more in a band-aid manner --- so does the 5DIII do any better?). Getting to 80 MB/s could get you to 3 fps continuously.
One sill thing: there seems to be no support for writing different images to the two cards, speeding up write speeds. Is there some hardware reason for this? This would seem to be the obvious way to go.
I suspect the Lexar Professional 1000X CF card still has faster in camera write speeds the the fast SD card I'm considering, the SanDisk SDHC Extreme Pro Class 10 UHS-I, though I've not seen any in camera write speed data for this SD card. Officially, the Lexar CF card has (official) 'min' read speeds of 150 MB/s, while the SD card has max read speeds of 95 MB/s...these are marketing numbers, but they still suggest a speed differential of around 2 fold, which lines up with my not well supported expectations (and with price differentials). Which suggests I do want to buy a CF card and write exclusively to it, using the SD card, if I get one, as backup and overflow. Or just get organized enough to carry to a 2nd fast CF card? But it would be cheaper to have just one fast CF card, and then to copy files to the SD card to free up the fast CF card...I can see accidental file deletion doing this under time pressure.
So it is probably one 32 GB CF card and one 64 GB SD card. This should get me through a weekend of shooting even if I don't get to laptop to clear the cards -- if there is no laptop around I'm probably not speeding all that much time shooting photos.
The remaining question: any point in getting a fast SD card? Or should I economize and get a slower SD card? And get more space? I do see 128 GB is the largest size for SD cards, and those are still fastish newer high price cards (i.e. you can cut speed by ~50% and get twice the size for the same price -- which might be the right way to go).
Update #1: assuming 40 MB RAW files, which is probably on the high side, a 32 GB card gives me 800 shots. A 64 GB card 1600 shots and a 128 GB card 3200 shots. So I'm facing 32 + 64 GB for 2400 shots or 32+128 GB for 4000 shots. 4000 shots will cover me through a 3 day weekend without laptop access (but with the need for battery recharges even with one backup battery -- under ideal conditions a battery will get you 950 shots, so that's one day on two batteries before I need to recharge).