With the PRS-505, you can use both at the same time. I have used both a 2g Memory Stick duo and a 4g SDHC card and they worked fine together, though it really is a rediculous amount of memory space. Several others warned about losing battery life just by leaving a mostly empty SD card in the slot, but honestly I didn't notice much difference. I finally removed the SD card because even with about 1,000 books, only a fraction of the memory stick is used.
