The PiDrive is just a mechanical hard drive with a native USB interface. It's a cool product at a reasonable price, but a normal portable USB hard drive will do basically the same thing. The only difference is the packaging.
If you want more power efficiency and performance, then an SSD is certainly an option. An expensive option, but one that could also offer a smaller overall package, since SSD drives are available in smaller form factors like mSATA and M.2/NGFF. Also note that performance gains will be limited by the USB 2.0 interface, so you won't get the huge jump in performance of a typical PC with native SATA, but you should still see some improvement.
You could use something like this for mSATA
Or this for M.2/NGFF
Pictures are links to Amazon USA (you should put your location in your profile for more relevant answers).
Found another one that supports USB 3.0. Of course the current Raspberry Pi computers are only USB 2.0, but this may change in the future, and this adapter will be ready. This also looks like it won't interfere with cooling as much, since it sits mostly over the USB end.
I just ordered one and will post about it after I have had a chance to play with it.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?