512 + 512 = 1024. I've never heard of it being slower because of using two sticks as opposed to one. Can you explain a little more on how you came to that conclusion? If you have dual channel DDR support, 512x2 will go FASTER, but if it isn't there, it's not inherently slower.
In addition, 2x512 will have better timings than 1 stick ... this is a laptop though, so I'd think it's probably a 1GB stick they would put in, but god only knows.
In the big picture though, I would not go with the extra 1GB of mem0ry
