For the most part, there three types of stores you can buy your next phone from. [for the following I will be using Verizon as the examples but they work for all the major carriers]
The CORPORATE store is one that is owned solely by the carrier. They have strict guidelines that all of their salesmen have to follow. All of the commercial promotions will be upheld but all of the prices that are listed will not be negotiable. These salesmen work on a hourly plus commission. They have goals for the month that can include: insurance plans, accessories, tablets, hot spots, android phones and iphones. As well as how many upgrades and new lines they have to get. This means when you purchase the phone you will be invited to get insurance and an accersory bundle you might not need. I recommend that your purchase your next iPhone using this option.
The 3RD PARTY store is one that is branded as "Verizon" has the labels on the front and all inside the store but it is owned by a company other than the carrier. Trade-in specials do not generally work here, any carrier gift cards are not excepted here and when making payments to your account in the store you are charged for this service. They normally have their own pricing structure and you will not find prices listed on the phones. These salesmen normally work on a straight commission ticket meaning they will give you all the time in the world to make sure you are happy but at the same time they are looking to make the most as possible. When transferring your number it is highly recommended that you open a new line try the service or phone and when you are finally happy with your decision change your number over. On occassion when you transfer your number and start with an android and try to go to an iPhone you can loose your number. If you are looking for a middle of road or low end android you can get the best price here. BUT it is very important that you get a guarantee that the phone they are giving you is NEW and not returned. At these locations, salesmen tend to use returned phones as "NEW" to make additional money as these returned phones have "spiffs" that make them more commission.
The BEST BUY option. Here I will be lumping in Wal-Mart, Best Buy and any other major retailer that sells cell phone service in their store. These salesmen are working on an hourly plus incentive based bonus. They will be knowledable of all the carriers they service but will not be as in depth on all the little nuances of each carriers plans. When they tap into your account through their POS, a frequent occurance is that your plan will get screwed up. If you have a grandfathered in plan I HIGHLY recommend that you stay away from this option. Once you grandfathered plan is gone, it's gone. Buying a phone here is not a bad option but be careful to make sure your plan doesn't get changed. Prices at these locations will be set and non-negotiable. However, you CAN find a better deal here sometimes [especially during the holidays] on your next phone.