How to choose my SD card type based on transfer speed?
You will likely not find an SD card which will outperform the flash in your phone, however there are substantial differences between memory cards.

Generally memory cards are labeled with a "Class" - which equates to the minimum speeds of the cards - Class 4 cards can handle 4 MB/Sec, Class 10 = 10 MB/s, UHS Speed 1 10 MB/s and USB Speed 3 30 MB/Sec.

Even within the classes though, there are huge difference in read and write speeds, and a good Class 6 card could outperform a bad class 10 card.   There is a good section on speed ratings in the Secure_Digital Wikipedia article.