I'm thinking of getting a new hard drive since the one I have is like 8 years old but it still works well. I don't know whether I should get a new one or just an external one to store files. Opinions?
I'd get both. A new internal drive, and an external drive to use for backups. I personally wouldn't use an external drive as my only drive - just incase I took it somewhere and it was lost or stolen.
external drives are convinient to use... they can be carried anywhere... I'm planning to buy one for my own..
Internals are cheap - I'd go for a new internal actually and just add it in one of your empty drive bays.
I like the both idea as well, but if I had to choose between the two I'd say internal. Clone the existing drive to the new drive and then use the old drive as a backup or something along those lines.