Are you currently having problems with your HD? I would not mess with it if there's no problem.
Best bet is multiple internal hard drives if you can, but as your laptop only holds one I would go for the fastest hard drive you can install. I thought I read somewhere that there is not much advantage of SSD for streaming audio or that the improvement is not equivalent to the additional price. I could be way off on this though.
I personally would not go the external route, especially if you are using an USB interface. It may be ok if your HD is on the USB 3.0 controller and the interface is on the USB 2.0 controller. Still, not as reliable as SATA and it can be hard to get the HD cash size and other specs of externals sometimes.
I'm having zero problems with my 5400rpm internal HD on the laptop. But, I am not using any VSTi or samplers at all. Just audio projects with about 30 or less tracks.
Let's see if someone more knowledgeable jumps on here to confirm or correct me.