If you are talking about the US, the Apple Store or Apple.com is the only place where the the sim free model is available.
The sim free model is not carrier-branded and will work on any carrier that permits it on its network. It has the Qualcomm modem and therefore can work on both GSM and CMDA carriers.
The Verizon model is "unlocked" however if you buy it at a retail store such as Best Buy it will be subject to US Resellers Flex policy. That means if you activiate it, for instance with a TMobile sim, it will lock to TMobile.
So if you want the greatest flexibililty and the fewest complications, the Apple Store or Apple.com is the place to buy and the sim free model is the most versatile.