Which is faster hardrive, ssd or external usb harddrive?

I am buying SIlicon Power 120gb ssd on Amazon at $63 dollars and was wondering if I am wasting my time and money.
After all, ssd is basically a usb stick and that got me to thinking... What is the difference in read and write speed of each.
Update: Sorry I worded my question slightly wrong, I meant to say at the end "USB stick" not " usb harddrive". If it is large enough to install an OS on then what is to keep a USB STICK from running as smoothly as an SSD? As its main purpose is to transfer files or hold them since the motherboard and... show more Sorry I worded my question slightly wrong, I meant to say at the end "USB stick" not " usb harddrive".
If it is large enough to install an OS on then what is to keep a USB STICK from running as smoothly as an SSD? As its main purpose is to transfer files or hold them since the motherboard and devices inside trhe desktop do the processing and such.
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