About the only thing I can add to what's been said is that there is generally a significant difference in the power requirements for higher RPM drives. The difference is sufficient that you can take it as an "explanation" for why most laptops deliver with the slower 5400 RPM HDs installed, since they all want to claim the longest battery lie ... life ... they can think up and are afraid some test lab might catch them if they exagerate by too much.

If you intend to operate frequently off the battery, you'll see somewhat shorter operating durations before the battery starts to fade with a faster HD; but it probably won't make much difference if most of your use is "plugged in." You can use "sleep" and "hibernate" modes to minimize the effect to some extent, but it can be hard to tell just how much difference you get that way.

Power consumption ratings generally are given by HD makers, although you may have to dig a little to find them ... if it matters to you. Whether what's given is accurate enough for you to consider comparisons significant in your choice is up to you to decide.

John