Guys, please understand that these memory cards are NOT for your average phones / devices. These cards are for HD / 4k video captures as well as photography shoots. I work with HD video and 4k occasionally. To put things into perspective, 1 min and 5sec of 4k video = 32gb of data. A 1080p timelapse of 5 mins @10x standard time = roughly 10gb of RAW files (individual pictures).
In a phone you won't see as much of a difference between the speeds. Mainly if you're moving massive amounts of data such as your entire music collection or a few movies. Usually better to do that via a card reader, although phones are getting faster transfer speeds. Most devices built in the last 3 or 4 years will be able to use any of these. SDXC was "introduced in 2009" so yeah 3 or 4 years and you're good all the way up to 2TB. As for why I would want one of these, mainly for my cell phone, because it's a lot easier to retrieve my data if my phone gets smashed, water logged, or inoperable by just pulling out the sd card. :)
The SanDisk Micro SD Extreme PRO 64GB UHS-I/U3 Micro SDXC is much, much faster than the SanDisk Ultra 128GB MicroSDXC Class 10 Memory Card (most voted one). It is more than 3 times as fast! So I would pick the SanDisk Micro SD Extreme PRO 64GB UHS-I/U3 Micro SDXC. You get half the storage, but 3 times the speed. And who needs more than 64 gb of external storage??
A lot of new hardware doesn't support expandable storage space. This becomes a problem if you have, say, a very large collection of music, where few modern mp3 players actually have SD card slots (let alone support for SDXC). Granted, I believe there are a lot of modern android phones with SDXC support, but then you're given sub-par battery life, compared to dedicated products like the classic iPod models which can easily last a couple weeks before needing to be charged, assuming light to moderate usage. Anyways, I think modern consumer electronics are trying to move towards bare-minimum onboard flash RAM storage, and then encourage you to just stream/host all of your media through a cloud service.