If paying full Apple store price up front, I would never buy anything but a SIM free unlocked iphone, which means generally buying direct from Apple Store. This gives you the ultimate zero-hassle flexibility.
Sometimes at retailers like Best Buy, Walmart, etc, you may find a discount price (vs. the Apple store price). But that, and even when there is not a discount, comes w/ strings attached... The hassle of getting carrier to unlock should you want to change carrier. Or if you take a trip internationally, and would like the flexibility to pop in a local prepaid SIM card.
Be aware that the Apple Store also does the same thing, in USA anyway, by offering phones set up for and with SIM card of one of the carriers. For same price as unlocked iPhone price. Never understood why anyone would choose that (vs unlocked phone) in the Apple store, if price is the same.