What is the real-world difference between 5400rpm and 7200rpm drives?

It is faster because it is new. After a couple of months you will not notice much of a difference. The faster drive will drain the battery quicker and generate more heat. It will be initially be marginally faster while reading from the drive but the computer will write to the drive via a buffer in the memory and so there will be little difference in write timings.

First off the faster drive will be faster. But before you change them, know that just having the same OS and type of computer doesn't mean the old one will work in the newer one. It was formatted to work with the chip set and Raid in the old computer and if the new one is different it won't work at all or not very well.

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Yes it will be slower. There are multiple reasons. First the new hard drive is simply spins faster. Data moves past the read/write head at greater speed. As it is newer it likely has greater data density per platter. Meaning more data moves across the read/write head per rotation thus speeding things up. It likely has a larger disk cache increasing the efficiency of read/write operations.

Now the question is why do you want to use the old drive?

- Data? You can transfer your files from your old drive to the new one. You just need a USB to SATA adapter and change some file ownership settings.

- Windows and Apps? Won't work OEM Windows licenses do not transfer and your new and old laptops likely have different version of Windows. Even if the license would transfer (retail license) transferring a Windows boot drive from one computer to another without nearly identical hardware is plagued with problems. There are many steps you have to take to prep an installation for transfer on the original computer.

The hard drive is old. That also means it has a greater likelihood of failure soon.

It would depend on your overall hardware and etc. But from my experience, the speed differences are quite noticeable.

It completely depends on the actual drives. The RPM's mean very little, as speed is actually derived from data density on the platters. A 5400 RPM hard drive can actually be faster to move data then a 10k RPM hard drive, because the data density is that much more efficient. Honestly, we will be seeing the last of the 7200 RPM's in the near future. The 5400's take less energy and have a higher density data structure.

Instead of buying laptop you should have bought a hdd enclosure, this would have saved you from the dilemma of 5400 or 7200 rpm

It's slower, if you want to store data (film, document etc) a 5400rpm is ok
but if you want a good performance for gaming or other intensive program that require a lot of computer power
a 7200 is better, as it will transfer and process data at a faster rate
but if you could afford a SSD drive i't way more better