I have been looking at the same two scanners for a while now; I finally went out and bought the LiDE 30 last night. For one thing, it was the only one available in stores around here. Besides that though, the only difference in specs that I could see is that the 30 uses a USB 1.1 interface, while the 50 uses a 2.0 interface. The 30 is slow, especially with scans bigger than 300dpi, but I'm not sure if that is a function of the scanner's hardware, the USB1.1 connection, or a combination of both.
One other thing that I have noticed is that the scan blurs really fast if what you're scanning is pulled away from the glass - such as a curled/upturned corner on a page, or the writing closest to the spine of a book. From what I understand, this has something to do with the type of image sensor (CIS in this case) that the scanner uses.
Nate