This depends. For some, paying for an iPhone in installments is a lot more practical than forking out a large sum of money upfront. That said, if you opt for a 24-month mobile contract, you’ll likely pay out more than the phone is actually worth over those months. This is because your contract will also cover your data, texts and minutes allowance, plus network overheads.
To that end, it pays to have a think about what’s more convenient for you. If you can afford to pay for the phone upfront, purchasing a SIM only contract will work out cheaper in the long-run. But if you can’t, then you’d be better off researching pay-monthly contracts that are within your budget.