HDD vs SSD vs USB

I'm building an HTPC. An SSD would be great, but they are very expensive and don't hold nearly enough data for my media library. So I thought I might be able to run my Openelec OS over a 32 Gb USB thumb drive and store my libraries on regular HDDs. Is there any significant speed benefit to doing this? What are the pros and cons? What are typical read\write speeds for HDDs (7200), SSDs, USB 2.0 and 3.0 thumb drives?