Internal and always keep a flash drive handy to back up your documents as you can always take that around with you to school and work. I bought an external hard drive from Best Buy last year and it's still in the box, lol.
i like internal because it is faster... but also safer (around me), i have already dropped an external once or twice and im not all the anxious to do it again lol
It all depends on usage. Internal will obviously give you more storage on your computer/laptop. This then enables you to work with larger files. Externals are beneficial for backing up data or for making your files more mobile. External is quite handy for DJs and media content creators as wel.
Best internal hard drives are the Solid State Drives, they boot and load up faster and make applications start quicker, new technology SSD is the way to go now
external for sure just less drag time on the laptop and now with the new bluetooth hard drive nobody can steal the files anymore except the dam HD itsself
Redundancy is key. Whatever you put on the inside should be on the external. You can have the fastest drive but if it goes then its pointless.