With O2 you get a Refresh contract which means once the device is paid off you can upgrade.
You also have access, if eligible, to O2 Open Guide: O2 Open & Friends and Family discounts - How to sign up
Bottom line, you have to go for the deal which is better for you. No-one here can tell you which to go for. I'm with O2 as my phone is paid for and I stick with sim only deals which offer some great deals.
Veritas Numquam Perit
Agree on this point; it used to be that, despite the rigmoral, you could get a decent reward for being loyal to O2. Now you just get the same deals as a new customer. Sky are likely to tempt me away this time round.
I'm due an upgrade.. I have had 3 contract phones with 02 for 20+ years, all latest phones. Costs about £140 p/m. I have been through retention and been offered Samsung S20 plus with 30gb data.. For £42 over 36m..... I've found the same phone with 130gb data for £44 p/m over 24m (new contract not an upgrade) .... I suppose my question is, how is a 36m contract better... Even when 02 tell me its an interest free loan for the phone... Its still dearer each month than a 3rd party through 02.. Loyalty seems to be long forgotten with 02.