To start with the internal drive will be faster than the external drive.
Are you up to fixing your own system here? Review this IFIXIT guide to see if you can do it: MacBook Pro 13" Unibody Mid 2012 Hard Drive Replacement|MacBook Pro 13" Unibody Mid 2012 Hard Drive Replacement]
There are three kinds of storage devices:
- Hard Disk Drive (HD/HDD)
- Hybrid Hard Disk Drive (SSHD)
- Solid State Drive (SSD)
HD's are the cheapest and the slowest drive you can buy. SSHD's are a bit more in cost but are faster than a HD. SSD's are the fastest drives but are the most expensive given their size. All of these drives are 2.5" laptop drives.
In addition there are a few different SATA standards. To make this simple your system uses SATA III (6.0 Gb/s) which is the newest version. Just make sure it states 6.0 Gb/s on the package or spec sheet.
I would recommend going with a SSHD if you need a lot of storage as its cheaper than a SSD and yet faster than a HD. If you can afford the cost, a SSD is a great solution. You may want to look at the amount of storage you need as the smaller drives are cheaper.
But keep in mind you'll want to leave 1/4 to 1/3 of the drive free for virtual RAM and application swap files no matter which drive you go with.
IFIXIT offers drives, so does other stores so depending where you are you might find a local store or may need to go online.