I guess a lot depends on why you think you need a power bank. Going outdoor camping for a few days, maybe?
I've never been in a situation where I needed a power bank to charge my phone... past or present. I instead have spare batteries on the ready. The way I look at it, is if you ever forget to charge your phone or you need to have a full battery for any reason, you can just pop in a fresh one and in a minute you're back up to 100%. With a power bank, you would still have to wait, and be tethered, while it charges. Samsung sells a charging adapter for the battery and so you can always have fresh batteries ready without having to charge them in your phone. My recommendation is get the charging adapter and a few spare batteries. However, if you have a good reason for wanting a power bank, then by all means get one. Just my opinion.
I do own a small, 4 cell,18650 battery power bank, and if needed, I could use it to charge my phone as it pushes 2 amps, but I bought it for my Mobius action cam when I go fishing.