Depends what you want to use it for.
If you want to put a particular laptop drive into a case (say, for example, a laptop drive which was displaced by an SSD), then there are lots of options, and few wrong choices. I would usually get one that was USB powered, because few modern laptop drives have any problems being powered over USB.
If, on the other hand, you want to be able to plug in different laptop drives at will, then you might want to consider a different type of device - most commonly called a "drive dock" - it's a thing that sits on your desk, and you can plug naked drives (usually takes both laptop and desktop drives) into it. Can be attached to a PC by USB or eSATA (mine has both connections). This is very useful as a diagnostic device, and much easier than assembling and disassembling an external enclosure every time.
There is another option. Seagate makes a range of 2.5" external drives called Go-Flex, which offer a variety of connections (I think it's USB2, USB3, FireWire, and eSATA) by way of optional cables. You can take a USB2 GoFlex drive and replace the cable with a FireWire cable, for example. Cute trick. I had a closer look at the cable, and the end that plugs into their drive enclosure looked familiar - it's a SATA connector (data + power). I had to try it, so I bought a naked laptop drive and plugged it in - it works. So if you want, for example, a USB3 cable for laptop drives, you can simply buy the USB3 cable for Go-Flex (they are available separately), and plug it into the drive. This works fine for diagnostic purposes, but you wouldn't want to use it for general carrying around, because the laptop drive is unprotected.
So like I said, it depends on what you want to use it for.