Which Hard Drive should i use for Disc Cache?

Hey community,

So recently I've been using a old slow HDD for my cache and its been bottle necking me too much.

My question would be, what would give me more performance?

a 1 TB External Hard Drive using USB 3.0 (the whole TB dedicated to just cache)

OR a SSD 128GB ? These will be just dedicated to cache..

Thanks in advanced !