Shhh... Nobody's going to buy my SD cards if you say it out loud...I'd go for a second CF card slot. Mixing card types, forcing the use of slower SD cards, is kind of useless. I'm rather bummed the 5D III includes mixed card slot types. Everyone who has tried to use the SD to save a second copy of the image written to the CF has experienced a significant loss in FPS.You mean two CF card slots?!
Damn... Want to buy a dozen SD cards?
That operate at 1/4 to 1/3 the speed of my CF cards? I can get up to 1000x CF cards, vs. 600x SDXC cards. Not only that, CF cards have a much lower latency, and buffer clear time with CF is three to five times faster (4-7 seconds for CF, 20 seconds for SDXC.) Even with the best of the best SDXC cards, they are still a slouch compared to even a moderate UDMA-7 CF card. I'll take the lower latency of CF at the same speed as an SD any day, even if it costs several times as much (although I'd say, given that I buy Transcend CF cards that have yet to fail me even once, the conversion rate is more like 3-4 SD cards per CF card, at mot.)
Seriously, didn't think there was so big a difference. What about camera size with two CF slots?
It might add another millimeter. Personally, I wouldn't quibble about that if it meant I could dual-write to a backup to a second card simultaneously, though.