In general, I find that the Monzo app user experience is so, so, so, so much better than the Starling app, which I find unintuitive and confusing. Also, Monzo’s budgeting and bill split/request functionality is much better than Starling’s in my opinion.
The big draw to Starling is the free cash deposits and the cheque imaging (I’ve had loads of issues using cheque imaging with Barclays, but non with Starling so far); these are two extremely useful banking features that Monzo doesn’t provide.
Overall, if I had to get a joint account with someone using either Monzo or Starling I would choose Monzo because I think Monzo is better as a day-to-day account because it is much easier to use and has better money management features (bill splitting, requesting, budgeting).
However, I would still (and do) keep a Starling account for cash deposits and cheque imaging since Monzo doesn’t have these features and I find Starling’s implementation to be better than legacy bank accounts - although the money form deposited cheques or cash is immediately transferred to my Monzo account.