It depends on whether you are just using it for music production or not. If you were putting the OS and programs on it then I would definitely go for an SSD, but for your purpose of removable media I would suggest a good old HDD.
Like Ocho Geek mentioned, the bandwidth of USB 2.0 will bottleneck your transfer rates and so the performance increase gained by using an SSD would be negated by this. Same goes for FireWire which only has roughly the same performance as USB 2.0. The solution would be USB 3.0 either as a PCIe card or built in to the mobo.
But that is irrelevant as, in my opinion, an SSD is wasted on music production. What sort of music production are you doing - Is it MIDI or wave based? If it's MIDI then your choice of hard drive will not matter at all as reading/writing MIDI data is not intensive. Again, wave data will be fine too as it is loaded into a buffer first. With a buffer of 128 samples being the norm, modern hardware should have no problem keeping up with demand on the HDD.
How many tracks do you usually have per piece? If it's <20 then just go out and buy yourself a USB 2.0 HDD. Anything more and I would recommend getting a USB 3.0 HDD. If you have the cash then I would get a USB 3.0 HDD just to be on the safe side and also to accommodate any future needs.
Hope that helps
And if it means anything, my usual production machine is a 2005 Apple PowerBook with a 5,400 RPM HDD. It can handle simultaneous read/writes of 10+ tracks without breaking sweat, so whatever you get you should be fine.
[EDIT] Sorry you replied while I was writing that. If you are worried about reliability either make your own enclosure (USB - SATA interface, 3.5" HDD caddy, 3.5" HDD), or purchase a rugged pre-built external. Western Digital's are more reliable than Seagate's, but are a lot noisier. This sort of thing: http://www.amazon.co.uk/External-SH93-500GB-Shockproof-Waterproof-Material/dp/B002TIOZZM/ref=sr_1_4?ie=UTF8&qid=1313514268&sr=8-4. Only I wouldn't trust the HDD inside it.
While I agree with an SSD being more reliable, if you are using it for recording you are soon going to knacker it out as of course NAND memory has a limited number of write cycles.

RSS ENABLED ::
AV SECURED