Buy whatever suits you best. If you have breaks in your shooting style that allow for swapping cards, two cards may be the way to go. If you are shooting high paced events that don't allow a lot of time to swap card, or if you are good at losing small items, one card may be a better choice for you. I know more photographers who have lost pictures by misplacing a card after they swapped it at a high paced event than have lost pictures due to memory card failure.
Most problems with memory cards are caused by inserting or removing when the card is communicating with the host system. I always power down my camera before swapping cards, and never remove a CF card from a reader that is powered up. In the case of SD cards, being sure to use the OS to eject them before removal should be enough. Following those practices, I've never experienced memory card failure, SD or CF, in any of my cameras going back to 2005.
I use a 400X 32GB Transcend CF card in my 5DII most of the time and a faster 600X 16GB Transcend CF card in my 7D. I have another 400X 16GB card and a couple of 8GB cards in my bag as well. All are UDMA or faster.
In the case of my 400X 32GB Transcend card, the 16GB version has the same read speed, but a slower write speed. They were the fastest available from Transcend at the time I bought them. The 400X 32GB is fast enough for the way I use the 5DII, but the extras speed of the 600X 16GB card is needed to take advantage of the 7D's faster write speed. When using the slower card, the buffer fills up quicker during continuous shooting.