heatsinks

Should I use a 3mm heatsink for my portable to save space or 10mm so it has the same height of my fan and cause the most air flow through the heatsink than over?

I'm looking at different options for heat sinks right now for my n64p. These are what I'm considering: But my question is: Is there anything wrong with using the heatsinks that come with the n64? No portable I've seen has used them. Is there a reason for this that I am missing? EDIT: There...