I've got (had) Revolut, Starling and Curve.
Curve was the first one I signed up with, it isn't a bank, it's simply a layer above your cards which you add to the app. You then pay with Curve and while abroad no FX fees and the mastercard rate. You can 'go back in time' too so if you accidentally use Curve and you've got your debit card selected, you can go into the transaction and switch it, to your credit card. Of course, you lose all CC protection when paying with Curve as it's effectively a debit card. My initial use case was that Curve used to allow you to use amex. As a result of the Curve card being a mastercard debit, you could then naturally pay where amex wasn't accepted. This was removed a few months after they (presumably) used up all of the investors cash by paying the additional merchant fees. They are currently testing amex integration again, but it's slightly different in that a. it's only for a small pool of beta testers and b. it's actually in the guise of a top up, where you top up your curve card ~£200/month max or something like that and can spend that money.
Starling is now my bank, I switched from santander to them. As above, no FX fees, mastercard rate, their app is great and really clean/easy to use. There's no benefit in having both Monzo or Starling (it's no different to having say an HSBC account and santander account), it's whichever you prefer. I prefer Starling because until recently Monzo was a pre-pay (i.e. you topped it up), unlike Starling which is simply a bank, though naturally that has changed now I believe Monzo has a banking licence, too.
Revolut I don't use, but it's handy to keep as it's also now a full on bank backed the FSCS, which also allows you to store money in a variety of currencies. It's useful, I suppose, as if someone wants to send me EUR I can give them the details of my revolut EUR account and there are no fees/conversions etc.
My use case nowadays is:
- amex where accepted (for cashback)
- when amex not accepted > starling
- when abroad > curve followed by starling as whilst both provide the same, I can use curve to put all my purchases on my non-amex cc