Most stores that sell phones may have two varieties of products, those which are locked and those that are free from a lock. A lock is something that may not be applicable when you buy iphone unlocked. The locking process is relevant to those carriers that offer prepaid plans and units to consumers, units that are exclusive to these carriers.
Thus your SIM for the locked phone will be one that is produced by the specific carrier for the prepaid unit. This cannot be readily unlocked, but you might get discounts and freebies, plus a whole lot of other convenient features in use and for deals. The deals for these plans often get you free loads and other stuff.
There are people though who might prefer the freedom of choosing their SIMs or going and testing out any network they might want. The networks compete with each other in terms of getting consumers that buy their SIMs and load up from them. The competition can really make for the most attractive offers.
Some offer discounted internet use and others free downloads. And any time you get hear of a favorite show for instance being offered exclusively through one network, you will think your locked unit is not that good. Locking is already an established process that limits your messaging, calls and internet activity to one exclusive program.
That means where there might be more offers out there, you are stuck with the carrier program. Not that this is bad in itself, since you could choose a really good carrier or network that offers a lot. In this way you will have more benefits out of the unit that has a locking system, but when you want this changed you can try.
Locked Apple phones can only be reliable unlocked by the IMEI Unlock app. The store you bought one from will not often let you have the unlocking process or that the process will be more of a hassle. The IMEI app will give you a process with no worries or hassle when it comes to unlocking the phone.
There are two ways that phones may be locked. One is with the SIM card, where SIMs from other networks could not be useful. The other is how the locking process is done on the phone itself, which is harder to unlock, and you need experts and their apps to do this, because a DIY might damage your unit permanently.
That means there will be experts out there who can help you. Again, your service center may not be able to unlock a phone that is prepaid, and this will mean your choice between a one or the other. Here you might wish to study the features of both before you decide, since both have their pros and cons.
Your decision will depend on use, and for simple messaging and calls, a prepaid unit is good enough. With things like internet use however, some will prefer a free unit. This helps them to do SIM switching from time to time.