what is the difference between 7200 rpm and 5400 rpm?
Theoretically, 7200RPM is faster, but with a trade-off (more RPM=more noise)
Theoretically, a 7200rpm HDD should "always" be able to access your data within 8ms while a 5400rpm HDD could take upwards of 11ms, and it may well take multiple reads before the OS walks through the filesystem to find the actual file you wanted.
Curiously enough, often once the data starts coming out of the drive, both drives can maintain similar speeds (head sensitivity seems to be the limiting factor), at least according to the manufacturers. But in practice this means very little unless you need to copy large amounts of data from one HDD to some other (HDD/SSD) drive.
Between the noise, heat, and power consumption, I suspect the 5400/5900rpm drives might be better. Although often the 7200rpm are cheaper, and I'd probably take price+speed over noise+heat.
It depends, as those drive speeds have been common for 20 years, but HDD's have evolved quite a bit over that time. All other things being equal the 7200 rpm drives can be faster. But a lot of modern 5400 rpm drives use more platters to achieve higher speeds, so a 7200 rpm drive might not be faster than a decent 5400 rpm drive at all. You'd really have to do your research to figure out where things shake out between two specific drives, because there's really not a simple, always true rule to fall back on.
And I would say that hdd's in general are so slow that the difference hardly matters anyway.