Does RPM or interface make more of a difference when using an external hard drive? Would you rather USB 3.0 and 5400rpm or USB 2.0 and 7200rpm?
		
		
	 
I'd rather have neither, but a USB 3.0 5400RPM drive will transfer files faster than a USB 2.0 anything. USB 3.0, until you saturate it, isn't too much slower than native, while USB 2.0 is very slow to access files. If I can, I use a 100Mb network to back up files, FI, instead of a USB 2.0 drive, because it's actually faster, with all the small files in most user's profiles.
So, no, don't worry about RPM nearly as much as the interface. Density, RPM, and firmware tweaks can make a difference for the drive's performance, but USB 2.0 is a ball and chain.