Put together a spreadsheet including all the variables and see which one works out best for you over the period of time you intend to keep the phone. I have always bought unlocked because of the good value SIM only plans available in the UK. Unlimited voice and messaging is pretty standard but you got to carefully check the available data on most of those plans and compare it to what you actually use. If you desire to use your phone data plan with a laptop it is another important element since some plans do not allow it.
I was surprised for example that BT who now own EE had a better sim only data plan but did not allow tethering when I was researching SIM only plans a while back.