Memory Stick Pro Duo 8gb VS. 4gb for PSP?

Which one is more efficient for playing games, the 8GB or the 4GB memory stick pro duo (Sandisk/Sony)??

I have heard that the 8GB is much slower than the 4GB, if this is true, why?

And if i were to buy two 4GB memory stick pro duo cards, and kept changing them (in the PSP console) would this effect anything in the PSP (ie parts or speed of Memory stick)?

Thanks in advance, and any other information that is related to this topic would be gladly appreciated.