It appears the 70D will be using SD cards, same as the 60D.
The good news is they are relatively cheap. Less expensive than compact flash in the same total capacities, at least.
The bad news is that SD memory isn't offered in as high speed specifications as compact flash. With CF, you can get upwards of 90 or 100MB/sec write speeds now.
The 70D has a fairly high frame rate... 7 frames per second if I recall. For that reason, I'd look for a fast, Class 10 card. Maybe something that offers 45 or 80MB/sec write speeds.The faster stuff is more expensive, Canon might specify in the manual for the camera (which is available for download now), if the 45MB/sec is adequate.
The next question is how large a card you'll need. Personally I don't like to put all my eggs in one basket and prefer to use a bunch of more modest sized cards. I shoot RAW almost exclusively, which makes for larger file sizes. I have fourteen 8GB CF cards for use in my 7Ds... each of which can handle about 290 images. I have six 16GB cards I use with my 5D Mark II, which are good for roughly 500 RAWs per card in that camera. The 70D has a new 20MP sensor, approx. 10% larger than my 18MP 7Ds and slightly smaller than my 21MP 5DII.
If you shoot JPEGs instead of RAW, you can get a lot more images on a card. A lot of data is discarded, when the image is converted to a JPEG in-camera. Conversely, if you shoot RAW + JPEG, you'll get a lot fewer images on the card.
I'd estimate you'll get 30 to 33 RAW files per GB of memory with the 70D. Figure what you shoot in a typical day or week and divide by that, and you'll know what capacity card(s) you need.
It's up to you, whether to get one great big card and hope it never gets lost with everything you shot over the last day or week or whatever. Or, get several smaller cards to reduce your risk, but end up having to change cards more often during shooting sessions. I can change out and format a card in any of my cameras safely in about 3 or 4 seconds now, so I don't mind smaller cards and frequent changes. would rather minimize my risk if anything goes wrong.
What could go wrong? Well, once some years ago I carelessly opened the memory card door on the camera too quickly and corrupted a card that the camera was still writing to. It was an older model that was slower writing, and I was able to recover about 2/3 of the images on the card... still, I lost about 75 images I guess. At a recent shoot a friend borrowed a brand new 16GB card from me and filled it with RAW + JPEG files. He downloaded (copied) the JPEGs, then when we later went back to retrieve the RAW files, the card was locked up and we haven't been able to access it at all. Thankfully we had the JPEGs, though I'd sure rather have the RAWs to work with!
I also have a bit of a personal dislike for SD memory cards, in particular... They are so small they make me nervous. I can imagine how easy they might be fumbled and dropped in the dirt, or get lost, or left in a pocket and sent through the wash, or whatever.
On another forum, we had a case where someone found a sizeable memory card in the street that was full of shots from a wedding! No identifying info written on the card, but... Fortunately, the EXIF data told us the date and time the shots were taken, and a few images showed the exterior of the church so someone who knew the area was able to recognize it... The person who found the card contacted the church, got info about the photographer, and was able to return the memory card to her. Needless to say, she was very happy to get it back! (Note: it's a really good idea to write your name and at least your phone number on all your memory cards!)
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
