SIM only deals tend to be cheaper than a traditional mobile-phone contracts that includes the cost of the phone itself in the price. If you already have your own mobile-phone or are happy to buy one separately, moving to a SIM only mobile deal could save you a lot of money or if you are not sure that you will pass the credit check of the phone then go with a sim, it will help you increase your credit score because it will offer you great benefits at more affordable prices plus you will get the chance to upgrade it to a mobile-phone contract once your sim only contract ends.