Is there a noticeable difference (i.e. a minute or more) when installing from say a slow USB 2.0 stick vs a fast USB 3.0 stick (compared to max theoretical speeds)? I'm asking because I want to keep a stick with Ubuntu on it around for reinstalling and am wondering which stick to pick. Assuming the system has a SSD so no HDD bottleneck.
Even with an SSD, there will still be some variation. Depending on the rest of your computer, the technology of the flash drives, the speed of the controllers, etc, you might see no difference, or the opposite of what you might expect.
However, in most cases, USB 3.0 flash drives should be faster. I use a 128GB USB 3.0 flash drive for pretty much all my installations (I think it still has GNOME 15.10 on it). Installing to a normal SATA III HDD, the installation takes about 15 minutes. From what I can remember from using a USB 2.0 stick when my USB 3.0 wasn't available, it took a little longer to install, but not very much.
So yes, USB 3.0 is faster, but as long as you have a high quality USB 2.0 flash drive and fast internal hardware, the difference will be noticeable, but not terrible. As long as you can spare a USB 3.0 drive, use it. You'll get the faster speeds with computers that can support it, and USB 2.0 computers will still install at 2.0 speeds.
If you just want to use the Live Environment, use USB 3.0. Since there's no bottleneck from the hard drive, using the LE with 3.0 will be much faster than 2.0, since data is being read directly from the drive, and not being copied anywhere except RAM.
My experience (which doesn't include USB 3.0 flash drives or ports) is that the installation takes many-many times longer than simply copying the information to or from a USB 2.0 drive -- hence I would conclude that, above USB 2.0 speed, the speed of your Live media is of much less import than the processing time and read-write time internal to the computer for an install of Ubuntu.