Well I put all my books/documents on SD card only. I can't say for speed as I did not try the internal memory, though the PRS 600 is pretty fast IMHO even for PDF files. It is faster than the 505 anyway
An advantage of the SD card is that it is usable from any computer and I don't have to carry the cable. I can add new books from home with the cable or from work using by putting the SD in the notebook.
Concerning battery drain I cannot say, not tested enough.