SD Cards: Which one is right for you?

I was reading about the 5D Mk III today and came across something interesting from Jeff Cable's (excellent) blog. Apparently Canon didn't make the 5D's SD card slot UHS compatible. (UHS stands for Ultra High Speed - see this for specifics.)
This means if I put my UHS-I 95MB/s SD card in a 5D Mk III, the buffer will clear at 20MB/s. Not great.
However, this is old news - the original post is from June 2012. If you're a 5D Mk III owner and you haven't come across this yet, go buy a CF card :p
However, it did get me thinking. If it's not supported on a pro body like the 5D, it's not going to be supported by my lowly 550D, either. A quick test confirmed this - the buffer cleared in exactly the same time (6s) using a 90MB/s card and a 45MB/s card, meaning the speed limit is somewhere below 45MB/s. I suspect it's 20... :(
So why spend the extra money on fast SD cards? It'll take you less time to back up your photos. Or will it?
As I'm home for study leave holiday I don't have my desktop PC to edit on - I'm using my laptop, which is a few years old now, meaning it doesn't have any USB3 ports. USB2 ports suffer from the same problem as the card slot in my DSLR - testing both cards using HD Tune, they capped at a 33.2MB/s read rate:

SanDisk Extreme Pro 90MB/s Card

SanDisk Extreme 45MB/s Card

Just for fun I also tested a much slower class 4 card, which capped at 20MB/s, and took twenty eight seconds to clear the buffer.

SanDisk class 4 card

You're probably asking yourself what the point of all this is. It actually comes down to something very simple. More speed = more $$$. If you're not going to be able to use the speed, you might as well spend the money on having more space. (For example, for the cost of a 32Gb 95MB/s card, you can get almost three 32Gb 30MB/s cards. So, if you don't have a USB3 port on your computer, you might as well get the slower cards.) How do you know if your computer is USB3 compatible? Both the ports and the wires are conveniently coloured blue.

The wire on my USB3 card reader

On the other hand, if you do have USB3 ports, the difference is dramatic:

SanDisk Extreme Pro 90MB/s Card

SanDisk Extreme 45MB/s Card

Conclusions
It appears both cards slightly under perform their claimed data rate (the pro more than the extreme) but it is undeniable that, when using USB3, the more expensive card is indeed faster. (Side note: since I'm not a fan of shelling out huge amounts for SD cards, my 90MB/s card is 16GB and my 45MB/s card is 32GB. I'm pretty sure this doesn't make a difference to the speed but I thought I'd mention it for full disclosure)
So what's the difference between copying at 74MB/s and 40MB/s in real-world terms? RAW files from my 550D are about 20MB each.
Copying 500 of them at 40 MB/s would take 4 minutes and 10 seconds. At 74MB/s, this is reduced to 2 minutes 15 seconds. In contrast, my class 4 card would take 8 minutes and 30 seconds. There's clearly a fairly large difference here, so if, like me, you take a lot of photos and have a USB3 port on your computer, it might be worth shelling out for a faster card. Having bought mine I've since found out Lexar cards tend to be a bit cheaper than SanDisk ones and perform just as well. So if you want a really fast, large card I'd recommend this. Otherwise a quick search on Amazon will yield plenty of options - avoid super cheap cards though - I had a cheap Transcend card which failed on me after about 6 months of use.
If you're stuck with USB2 ports on the other hand, I'd recommend something like this or this (Links will open in new tabs). The 400x card is overkill, but since cards tend to under perform on their rating, you might want to pay the extra fiver to make sure you're getting top speeds. (That said, if somebody has the 200x card and can send me a test screenshot, let me know!)
 As always, if you have any questions or just want to say hi, feel free to leave a comment. It lets me know somebody is actually reading this :)

Thanks for reading,
-Benji