Deciding what card to buy for a specific camera in order to get the fastest performance of which the camera is capable without spending more than necessary on a card that is even faster than the camera is capable of taking advantage is a daunting task. Not only can it be very confusing when comparing speeds of various cards, but learning how fast a write speed your camera is capable of can be near impossible. Manufacturers rarely publish the maximum speeds at which their cameras can write to a sufficiently fast memory card.
The current methods of rating various card speeds is, to put it mildly, A mess. While the majority of cameras currently produced use some form of SD card (SD, SDHC, SDXC, and SDIO), many of the top professional models use CF cards. Even cameras that have slots for both types of cards often perform better in terms of speed when using a top rated CF card.
Part of the confusion among CF and older SD cards is the difference between read speeds, which is what most 133x, 600x, or 1000x ratings are based on and write speeds, which are what most photographers are concerned with since it affects the camera's performance when shooting large numbers of frames in quick succession.
With SD cards, the capacity of the card is the primary determination as to whether it is an SD, SDHC, or SDXC card. Base transfer speeds are required to be higher for SDHC cards than SD cards using base speeds, and also for SDXC cards compared to SDHC cards at base speeds. But there is nothing preventing a manufacturer from producing a lower capacity SDHC (or even SD) card that is capable of faster speeds than the required base transfer speed of an SDXC card. With SDHC and SDXC cards transfer speeds are rated by the UHS-I or UHS-II specification, but write speeds may still be slower. Standards for SD cards designated as belonging to a certain Speed Class Ratings were developed to address the need for cards to be rated at particular write speeds as well as read speeds. But there can still be much variation between different cards of the same speed class rating as well as different speeds obtained from the same card depending on such things as the frequency of soft errors, file fragmentation, and whether a single large file or multiple smaller files are being written to the card. Also note that an UHS-I rating is not the same as an UHS Class 1 (U1) rating!
Several years ago one go to guide for Canon or Nikon shooters would have been Rob Galbraith's Digital Photography Insights. But the information there is becoming a little dated. The most recent Canon camera tested is the 1D mkIV. There were many more cameras that had test results listed at one time, but many of those cameras don't even appear on the drop down list any more and some of the cameras still in the drop down list no longer display any test data. (The page linked above does contain some good information regarding how memory cards work and how to identify a particular card). To the best of my knowledge there is no comprehensive database that tests a plethora of cards in each of many camera models currently on the market. I would love to be shown such a database that proves I am misinformed at this point!
So what can a camera owner do to find what the most efficient memory card is for a particular camera? One that is capable of the camera's top speed without being overkill? Short of buying a bunch of cards and testing them yourself, there are places to look to find some help.
I always start by reading the camera's manual. Even if the camera's write speed isn't explicitly spelled out, there are often clues. For instance, on page 61 and again on page 270 of the Canon 7D Instruction Manual (updated edition for firmware v.2 or later) we read the following:
"Figures in parentheses apply to an UDMA, 128GB card based on Canon's testing standards."
So we should probably consider an Ultra DMA (UDMA) compliant CF card.
My experience is that an internet search (google, bing, etc.) can also usually turn up something. Search google for "Canon 7D UDMA card speeds", and one of the top results is this link. A member has posted results of a test he conducted with his 7D and various CF cards rated at different speeds. Substitute 'Nikon D700' for 'Canon 7D' and you get similar results for that camera. As with most google searches, the more you know before you start, the easier it is to find what you need. Include 'UDMA-7" in the 7D search and the top result leads you to this discussion which makes it fairly clear that an UDMA-7 card is no faster in the 7D, even after updating the firmware to version 2, than an UDMA-6 card. So for the 7D, an optimal card would be a UDMA-6 compliant card.
Say you have a Canon T5i. What can we learn about it? Page 89 of the manual indicates a UHS-I compatible 8GB card was used to generate the best burst performance. Note that the burst performance increases with the UHS-I card when shooting JPEGs but stays at 8 frames when shooting RAW. Canon Europe's support site says a large capacity Class 6 or higher SD should be used when shooting video. So in the case of the T5i, the question would be whether you plan to shoot JPEG, RAW, or video most of the time. How you answer that would determine whether the UHS-I Class 10 card would be overkill or not.
As to how various cards of the same rating compare, reviews such as this one at tom's HARDWARE are fairly easy to find and show that not all Class 10 cards will out perform all Class 6 cards. Once you've narrowed it down, you can always check the reviews for a particular card at amazon.com. These reviews for the SanDisk Extreme 32 GB SDHC Class 10 UHS-1 Flash Memory Card 80MB/s (SDSDXS-032G-X46) are fairly typical. There are usually several reviewers who will post results of their test with a certain card. Some reviewers will demonstrate more knowledge and objectivity than others. The review rating system at amazon is useful for pointing to the most helpful reviews.
Note that as the use of high speed, high capacity SD/SDHC/SDXC cards become more ubiquitous, the prices have trended downward. This is probably a result of both higher volumes and increased competition among suppliers. The fastest CF cards, though, can still get rather pricey.
A SanDisk Extreme 32 GB SDHC Class 10 UHS-1 Flash Memory Card 80MB/s currently sells for under $50 US at amazon.com. A SanDisk 32GB Extreme Pro CF memory card - UDMA 90MB/s 600x currently sells at amazon.com for just under $150 US. The SD card writes at 80MB/sec, the CF card writes at 90MB/sec and costs three times as much. A newer UDMA-7 Sandisk 32GB Extreme Pro CF Card rated at 160MB/sec is slightly cheaper at $137 US, but a newer version of the 32GB SDHC Class 10 UHS-1 Flash Memory Card 95MB/s rated at 95 MB/sec is still only $61 US.
And as always, beware the deals on the top brands that seem too good to be true. The most expensive cards are also the ones most often counterfeited. The best way to protect yourself from buying a counterfeit card is to only buy from reputable sources that have a liberal return policy.