False limitations on false economies for a fallacious argument does not say much about your ability to consider basic logic.
You won't get decent performance out of an enclosure, and frankly all the people who talk about it above in this thread have as little knowledge of computers as the average comp sci lecturer.
Ok, so ignoring the fairly bad advice about enclosures, what is the best way to get performance from a drive ?
A powered desktop drive will be fastest, but a portable drive maybe acceptable for a laptop backup while the laptop is being charged or has sufficient battery life.
If you want no delay from a shared bus, then thunderbolt is the way to connect, otherwise if you don't mind wasting several hours on a backup, then USB 3 is the way to go. (Not a good option if you have a laptop, but USB 3 is fine if you have a desktop.)
Given that the OP clearly wants a fast backup for the entire system, a thunderbolt powered desktop drive would be best, followed closely by a thunderbolt portable drive if connected to a laptop that shifts location frequently.
And given that the OP is considering a SSD, then it is around the same price for a dedicated thunderbolt HDD which will significantly outperform the SSD on USB 3, along with a larger capacity drive.
It is just a question of how much patience you have to wait to backup.