San Disc are the best, they can repair themselves if corrupted, the 5D supports UDMA cards and can make use of their extra speed, the burst frame buffer expands from 78 Jpegs to 310 with UDMA, sadly no difference when shooting Raw.
There are only 4 memory chip manufacturers in the World and 3 of those are San Disc or subsidiaries. The other one is Lexar. All are in Korea.
If the cards you have are genuine then you have the best cards available, they only put the San Disc name onto top end chips, lesser chips, either in speed or capacity, get sold off for pennies to other manufacturers to package. Other good manufacturers such as Kingston commission production runs from San Disc and then package the chips in their own factories.
What San Disc offers is a re-programmable control chip on their Extreme brand, all memory cards have pages of memory, they all start off as a 256Gb card, but only a tiny minority have all the pages working, most fall short of the spec either in capacity and/or speed, the control chip uses those pages that are tested to work at the speed the card is classed at the number of working pages determines if that card is a 4Gb, 8Gb etc., and the chip is packaged accordingly. The point being that there is always fully functioning spare memory pages, Extreme cards can re-program the control chip to use one of these if a memory page gets corrupted. It does this in the POST (Power on Self Test) test when you insert a card or turn your camera on, you may notice the memory LED lights up for a few seconds whilst it does this POST test.
The only other card that can do this are the Lexar Gold series.
A possible reason why two files are corrupted is that you have erased an image from a full card and taken another image. All images use compression (even Raw files, the only exception are DNG files which are always the same size as the sensor, in the case of the 5D 22Mb), this means that the file size varies with image content, it's possible that the new image is larger than the deleted one and has overwritten part of an existing file which will corrupt both.
Stick to San Disc Extreme or Lexar Gold, they are the best cards most secure cards available.
To be honest I have never had an Extreme card fail (touch wood). But it obviously makes sense to use multiple cards, the 'all your eggs in one basket' approach, I don't own any cards larger than 8Gb for this reason, I tend to use loads of 4Gb cards, but they can be a pain when you need to change one mid shoot.
Always format the cards in camera after your images have been saved securely onto your hard drive (preferably two), this writes a new DCIM (Digital Camera Image Management) file which is how the camera keeps a record of where on the card each image begins and ends. Just deleting them doesn't do this it uses the existing DCIM file which might well be corrupt. ALWAYS format the card in camera to start afresh with your next shoot.
Chris