There are xxx things that affect the recording potential of a camera:
* JPEG or RAW or JPEG+RAW - per frame, JPEG is always much smaller than RAW, and can be more so if you choose the lower resolution options. RAW is always much bigger, RAW + JPEG capture is the largest of them all
Trade off: potential image quality from RAW vs. image size from JPEG
* sensor resolution (not size) - a 20Mpx sensor has 4x as many pixels to record as a 10Mpx sensor (inverse square law it you want to know why) Trade off: at typical 300 to 360 dpi printer resolution, ability to enlarge and / or crop an image to a size you want to view. More pixels. more crop or enlargement potential (that pesky inverse square law again.)
* single or continuous shooting: with a single frame, the exposed image is saved to the high speed camera body memory, then transferred to the card. If you shoot a lot of continuous frames, the buffer fills, the camera stops shooting, the red light comes on while the buffer is emptied to the card.
* the inbuilt 'speed' of the camera record hardware to card. With an SD card it appears that most current cameras use relatively modest transfer hardware - on the bodies I've tested it looks similar to USB2.0 (hardly rigorous testing but I can copy stuff on my USB3.0 laptop much faster than the same volume on a camera.
* the memory card performance. Check the write performance - who cares if it takes X minutes longer to transfer to the PC, its how quick it copies from the camera to the card.
There are other things of course but these are the major ones that affect my use - I have an EOS 6D, 20 Mpx senso, shoot RAW only, often continuous shooting (birds, aniimals, and grand-children (same characteristics - won't pose, highly mobile, etc etc
I have tested loads of cards over the years and like most people don't want to waste money - although it never ceases to amaze me the number of people who buy a body and lenses costing £100's to £1000's then use cheap generic cards.
I have also tried to standardise and 'future proof' - stndardise cos I don't want a collection of cards for differnt uses such as phone, GPS, tablet camera. Future proof in that every time I buy a new one it is the best and most flexible at that time for size, performance, and function. And I test every one from end to end when I get it.
So what does that mean? I standardised on micro SD cards - with an adapter it fits every SD/ CF device I own. I only used branded cards, from reputable suppliers, that have clearly stated performance characteristics.
On A PC with USB3.0 interface, I test each card end to end for Read and Write with H2testW using a branded high speed SD card reader (the difference between cheap and branded can be significant.) Again on the PC, I test for writing performance for file sizes of RAW files that are typical for a Canon EOS 6D (25 to 30 Mbs) using ATTO DiskBench mark. Finally to get and idea of camera buffer capacity, I set to continous shooting, and shoot a watch face second hand until the camera skips a shot.
I recently needed to get some new cards and decided on 64GB with the advent of 4k video (don't do it, but who knows) so tested 3 of those I have used in the past. Of those I tested - FOR MY TYPE OF USE in camera and other devices - the Sandisk Extreme Pro was fastest with 100Mb/s write. The Extreme Plus was 80Mbps, the Lexar x633 30Mb/s, and Samsung Pro 28Mbs. All of them miles faster than the EOS 6D buffer because they all recorded 18-20 frames before they skipped a frame (at just under 6 frames a sec quoted by Canon BTW.) And any of them will be just fine, or slower cards still if you typically shoot single frames in JPEG. I got the Sandisk extrem plus because the price difference of the extreme pro was not justified for my typical usage.
To finish this long note off: I'm not obsessive about this - it only takes a short time to check on the occasions I buy cards and I have never had a failure since doing this. Don't buy cheap cards, test the ones you buy end to end, if you do something more demanding - RAW, continuous shooting, then test write speed for size of files your camera will create, and perhaps the size of the camera buffer.
In closing, I must give credit to the now unknown writers of numerous articles on memory performance I have read over time to do this testing.