You might get more images in a long high speed burst, and, if you have a fast USB3 reader, images will download faster.
I only upgrade in the event that I go for a larger card. I just upgraded from three 60mb/sec 16mb cards to a 64gb 1000X card but its mainly because downloading of a large number of files will be faster.
Since I have dual cards now, I have a built-in backup so large cards are not a concern.