the RPM on the hdd's give you a basic idea of how fast the drive can read/write. I'm assuming with the 2nd hdd that the 5400 is the lowest speed it'll ever run at, while the highest speed is 7200 (which is pretty standard). Iuno, seems kinda a weird thing to have to me, but maybe I'm missing something.
So to answer your question, taking the above as true, I think you wouldn't notice anything. The drive would idle at 5400 RPM, then when you wrote to the drive it'd speed up to 7200 RPM.
That's just a best guess tho, I've never seen a hdd with two speeds on it.