After removing HD, use it externally as USB 2 or USB 3?

Theoretically yes, but in true practice no. This gets into the drives I/O spec and what the drive is truly able to perform.

So if we look at the SATA III interface we can theoretically achieve 6.0 Giga bits per second which is faster than the USB 2.0 theoretical limit of 480 Mega bits per second and if we look at USB 3.0 which offers a theoretical limit of 5 Giga bits per second we can see it still is short by 1 Giga bit of throughput!

Now in the real world most USB 3.0 Thumb drives max out at 800 mega bits per second (100 MB/s) And most HD's can't sustain that either. Yes, some of the newest USB flash drives on the market now are pushing 1.6 Gb/s (200 MB/s) But you won't find any HD's offering that. And yes, SATA III SSD's will offer much better performance so a USB 3.0 case would make sense for it.

I would save your money and just stick with a USB 2.0 case for your HD.

FYI: Becareful with the B's! - little b is bit, the BIG B is Byte.