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As many people know that microSD card can be cheap or expensive, depending on how fast the card is, what the brand is, how the card is manufactured, and the size of the card. MicroSD card currently ranges from anywhere of 4gb to 256gb and could cost anywhere from $5 to $100+. This post is more of an informative post, as I am just trying to explain some of the differences.
There are different class of memory card. Most of the cards now are rated as Class 10, which means that it will just meet the minimum requirement of the read/write speed that has been classified as one of the standards. Class 10 would mean that it meets the minimum of 10MB/s read or write. There are other classes, which are 2, 4, 6, and 10. Also, there is UHS classes, 1 and 3. UHS 1 means the card is capable of minimum 10MB/s. UHS 3, as you can tell by the numbering, it will be capable of a minimum of 30MB/s.
There are different labeling for SD cards, such as SD, SDHC, and SDXC. The regular SD standard don't really exist much anymore as the maximum capacity of a card was only 2GB. The SDHC standard capacity is anywhere from 2GB to 32GB. The SDXC standard is 32GB and up to 2TB.
For showing examples of differences of cards, I will be showing Samsung microSD cards.
Samsung has a variety of range of microSD cards and their naming are: EVO Select, EVO, EVO+, Pro, and Pro+
EVO has the rating of UHS-1, which means it meets the SD standard of minimum of read/write of 10MB/s. Samsung states that the card can reach up to of 48MB/s. When a manufacturer doesn't state the up to of which section of the card that can reach that speed, you can just assume that it is the read speed that can reach up to of 48MB/s (or whatever the manufacturers state). The write speed, which might be important to some that writes files to the microSD card often, might be slow, around 10MB/s or so, so that it might take time to transfer files onto the microSD card. The card is good for storing media, should be good enough for phone usage and regular cameras.
EVO Select has the rating of UHS-1, which would also mean that it has a minimum of read/write of 10MB/s. From the spec of the card, Samsung states that it can reach up to of 80MB/s read and 20MB/s write. The key word is up to. That speed normally won't happen constantly, maybe a burst at times, but that card should be good enough for people who want to store media files and use it for their phones and camera (though I would not recommend for DSLR burst shooting).
EVO+ also have the same rating of UHS-1, and going off of Samsung's specs, it is pretty much identical to the EVO Select, having the same speeds of up to 80MB/s read and 20MB/s write.
Pro has the rating of UHS-3, which means that it has minimum of read/write of 30MB/s. Samsung says this card can reach the speed of up to 90MB/s read and 80MB/s write. Samsung's Pro series cards are one of their top cards available for consumers and have pretty high ratings because of it's fast speeds and some just prefer having cards with faster read/write speed for their phones or cameras to have faster access to files and saving them when compared to slower cards. It is also great for 4K video recording on most devices, though slower cards like the EVO series can record 4K from phones also, but some cameras won't accept anything lower than UHS-3 when doing 4K recording.
Pro+ also have the rating of UHS-3. This is Samsung's fastest card as far as I know, stating that it can reach the speed of up to 95MB/s read and 90MB/s write. As state above, it is great for those who wants one of the fastest cards, for 4K recording, for use with cameras for 4K videos and burst shooting, etc.
What is the take away from this? Choose the cards wisely on what you think you want to do with it. Are you planning to use the expandable storage on your phone as many a storage for media consumption (videos and music)? Then you might be fine with the basic card, EVO, which should be good for majority of the consumers that aren't looking for anything in particular but the price and storage size. Need a card that is great for DSLR, Action Cameras, etc. with 4K recording? Might consider the EVO+/Select, Pro, or Pro+, as some cameras do have some requirements for shooting in 4K, but if you were to shoot 4K from a phone, it might not need all that requirement and possibly the phone will want to record the video on internal memory first and then you could transfer the footage to the microSD afterwards.
All I can say is, look at reviews and possibly benchmarks on the cards to get a sense of how fast the card you're looking at that you're interested in buying. Not all cards are created equal, even though they all have a Class rating. Amazon might be a good place for you to look at for reviews and pictures of people possibly doing benchmark on the memory cards to show other consumers how fast the card might be. Random access of read/write speed is also important in some ways, but I did not cover it.
This post is based on my knowledge and some facts from other sources. I might not be correct with all the information and left some of the facts out also.