I personally don't care much which phones any GSM carrier offers. In most of the world, very few people get their phones from a carrier and are much better off for it. People in the US can do the same thing. I have a 1520.3 on T-Mobile. If the carrier sells the phone I want and it can be unlocked after a short time, then I might buy from them. I bought four T-Mobile Lumia 640s from WalMart but only because they were just what I wanted. Three were gifts and one my wife used a Lumia 925 and needed more storage. The Lumia 640 from T-Mobile was the cheapest WP from any source that gave her the storage and everything else she cares about (she's not into photography).
But even when a carrier sells a phone some people may be financially better off buying the same but unlocked phone from a 3rd party. If a customer wants accident insurance, the carrier will charge a fortune compared to what a retailer can sell a 3rd party policy for. I think for the last phones I bought from a non carrier, a 2 year zero deductible accident policy was 50 cents per month per $100 of phone price. The carrier will charge many times that and sometimes have deductibles.
The carriers will try to make it seem like there are drawbacks but some are bogus and all are exaggerated.
Example: the carriers use their leverage with manufacturers to be the front line for warranty work in a market. That means if you don't buy a phone from a carrier, you don't have a warranty from the manufacturer. But retailers have to compensate. Yes, its something you have to make sure of, but the best retailers offer an identical warranty usually free as included in the price. My 1520.3 has a 1 year warranty from B&H. And it did develop a bad case of phantom touch. They replaced it with a brand new one, with zero hassle because it was in the 1st couple of months. After that, they would repair it just like a carrier would repair a defective phone. The only difference is the carrier would ship it off to a factory center for the repair.
All a carrier really offers is convenience. If your phone breaks, you can drop it off at a local carrier store instead of mailing it to a designated repair facility. Dropping it off doesn't save time since they just ship it off anyway. Besides, who really can do without a phone for even a few days? So you need a cheap backup to pop your SIM in either way.
Another carrier carrot is carrier apps or features. An example is WiFi calling for T-Mobile. It lets you make a call over WiFi when you have no bars. Think about that. Just how many people don't get any bars but also have WiFi everywhere they don't get bars? So these apps are mostly gimmicks. Sure they can be deal breakers for a tiny few but not the vast majority of us.
So... figure out what phone(s) you want. If your carrier doesn't sell any of them, see if there is a variation of that phone that has the bands your carrier uses (retailers will be happy to tell you). Then buy the phone you actually want. Do you really want to live with 2nd, 3rd, 10th best for months or years because you didn't feel like doing a little research?
Almost forgot some of the other benefits (NOT!) of buying a phone from a carrier: Such as carriers often delay or refuse to allow OS updates. With a non-carrier phone, the updates come directly from the OS maker as soon as they are ready. Apple has enough clout to avoid the delays but you are out of luck with a WP or Android. MS has provided a way to bypass the carrier but its not mainstream and there is no guarantee the carriers won't stop it or make it so hard to do, normal people would never bother.