I need to buy some extra memory cards for an upcoming trip with my digital camera. Is it better to buy a large number of smaller cards, or a smaller number of larger-capacity cards?
The logic behind this question goes something like this: if I buy a single 4Gb compact flash card and it fails, I'll lose all my photos. If I buy 8x 512Mb compact flash cards and one of them fails, I'll only lose 1/8th of my photos.
The reality is that you can't compare the two options like that. If you have 8 small memory cards, then there's approximately 8 times the likelihood that one of them will fail than if you just had a single 4Gb card. The impact of a failure is lower, but the likelihood of a failure is much higher. Generally I don't think you're any better off with one option over the other, but you need to judge based upon your own circumstances.
There are other issues with juggling many smaller cards, though. With smaller cards, you'll keep finding yourself in the situation of having a card almost full. So you have to choose between swapping cards around now before the action starts or missing some shots while you're swapping later and things are happening around you.
Because of this you end up with lots of cards that are 3/4 full, and by the end of the trip you're juggling cards around trying to squeeze in the last few photos. With a single large card, you don't have these swapping problems.
It can be frustrating trying to find an empty card when you've got lots of small ones lying around. If you don't have a system to manage the cards, you have to put in the card to see if it's empty, and if it's not take it out and try another one.
Multiple cards can also be hard to manage when you're dumping images to a laptop or portable storage device during your trip. You need a good system to remember which cards have been downloaded and which haven't, or you'll risk wiping photos that haven't been downloaded yet. In my experience, this is a much larger concern than card failure. It's a heck of a lot easier to manage downloading with just one or two large cards.
Cards can be lost or misplaced when they're not in the camera, so you need a decent card wallet and a safe place to store them. With one large card, it's always in the camera and much harder to lose.
All the inserting and removing of lots of little cards causes extra wear and tear on your camera's card bay, too.
One benefit of having multiple smaller cards is that you can keep shooting if a card does happen to fail. You can also lend cards to other people or leave cards with camera shops for printing, too.
There can be other considerations too. I have a friend who owns a portable CD burner. He shoots with multiple 512Mb cards, since they fit comfortably onto a blank CD. 1Gb cards would have to span two CDs if they were fairly full.
Personally, I think the best solution is a compromise between the two extremes. I have a 1Gb compact flash card and a 256Mb one. I rarely take the 1Gb card out of my camera, but when I'm shooting a large event I can swap cards and keep shooting while the other one downloads to a laptop. With just two cards, it's fairly easy to remember which one has been downloaded and is safe to format. And if one happened to fail, I can keep shooting with the other one.
So I say buy a high-capacity card to leave in the camera most of the time. Then buy a second card of whatever size gives the best cost/Mb value, as long as it's fairly large (at least 1/4 the capacity of your large one). If you need more storage than these two combined give, buy more high-capacity ones. A smaller-capacity card still comes in handy if you want to lend it to someone, leave it with a digital lab, or stick it in your point-n-shoot camera.