It depends what you want it for?
If you’re going to access files regularly then 7,200rpm and USB 3.0 for when your PC’s replaced.
If it’s more for keeping a back up every week or two then a slower drive and less cache.
If you don’t need it to be portable, then you’ll get more Gbs for your money buying internal. Don’t be afraid of fitting a drive inside your PC. A 3 minute job. If you need it to be portable then a dock / internal drive will be cheaper although often the docks aren’t as high a quality as if you bought a purpose built drive.
I’ve got 3 x docks with internal drives in them (functioning as external drives) only because I’ve used all 8 internal sata ports!
Get Western Digital / Seagate. They have warranties far longer than many people imagine. My WD had five years. Posted it off and got an upgraded, new model sent back within a week.
If you’re storing important data then remember, the bigger the drive, the more you’ll lose. Losing 500Gb sucks. Losing 3Tb makes you want to cry.
If your drive’s dying, unplug it, wait until you have a replacemernt and you should be able to save a lot of its data.