Question regarding XMS2700 RAM modules

[galas]

Hey, I'm ordering some ram, and I was wondering what the difference was between their 2 types of XMS2700 256MB modules - looking at their product listing at http://www.corsairmicro.com/main/products.htm I see the difference is 'RAM Type':
CM64SD256-2700CX2H is 16Mx8 and
CMX256A-2700C2 is 32Mx8 - but what does that acutally mean? Is one better than the other?
Thanks for all the help,

.:galas:.