This is an old thread and I had answered a similar question in a more recent one. For others looking for a fast 64GB card I'd recommend the Sandisk Extreme Plus or the Extreme Pro. There IS a difference in performance between those and the cheaper cards for those who care about transfer speeds. Key to picking IMO is to find cards with high rated write speeds. The Extreme Plus line is rated by Sandisk as having up to 50MB/s write speed, 80MB/s read speed. Looking at their specs on Sandisk's site one will notice that they do NOT rate the write speed of their Sandisk Ultra Class 10 cards. Instead they'll just say "Write speed lower" when compared to their 40MB/s read speed.
Having one of the 64GB Extreme Plus and one 64GB Ultra I did some testing with USB 2.0 and 3.0 cables from a windows 7 workstation by timing file transfers to my Note Pro. Results are in the other thread linked below.
These are simple timed tests and they're clearly geared towards larger file transfers. Most memory will slow down significantly for random writes of very small files. For all I know the tables would turn in random small writes but since that's not what I do with external microSD I care not to test that.
Bottom line from my experience is that between a Sandisk Ultra (or other comparable Class 10 card) and the higher end Extreme Plus cards you will see significant improvement in transfer speed even on USB 2.0. For those like me who are impatient the nearly 2x cost upfront is a gift that will keep on giving for a long time down the road. My one wish is that they'd come out with larger capacity ones than just 64GB. I'm excited for the future when these faster high capacity cards come down in price and become the norm.