It depends on what you want to do and/or pay for personally.
If you are the kind of person to buy your phones directly from your carrier, there's always financing to get "better" options. All of the major carriers get the new flagship Samsungs, iPhones and usually LG phones too? Everything else is pretty varied. An example is the HTC 10. I can't recall which carrier was not interested in it at all, but the other three got it, even though T-Mobile recently dropped that.
My situation is kind of different from some others who use these forums. Me and my family are on T-Mobile. We just pay for service because all of us bought our phones outright. Some of those are T-Mobile branded phones, that you don't have to buy from T-Mobile directly, or just unlocked and works well. T-Mobile supports BYOD (Bring Your Own Device) nicely. We don't really have super expensive high-end phones though.
If you buy a phone from T-Mobile, you just pay an additional amount to your bill, each month depending on your credit and what phone you choose. They also have Jump On Demand, which you pay an additional month each month for, so that you can change your phone every few months. I'm not sure of the rules though.
Libra89 did a nice job of explaining the benefits of T-Mobile postpaid service. I also use T-Mobile and have a postpaid account with them. Like Libra89 said, the monthly cost that your device will add to your bill will depend on the device you choose. The device's total retail price gets split up into 24 monthly payments, but if your credit is less than what T-Mobile would like you to have to offer you the 24-month installment plan, they may require you to pay a portion (usually $50 - 100) of the device's cost upfront and the remaining balance can be paid off over 24 months.
The advantage of going postpaid (pay at the end of every month, instead of pay-as-you-go) is that often you will have the ability to buy a phone straight from the carrier and finance it through the 24-month installment plan (TMO calls this EIP, or Equipment Installment Plan) or finance it through JOD (T-Mobile's Jump on Demand program).
As far as I can tell, that is the only real advantage you get from going postpaid with a major carrier. The options I've described above also apply to the other major carriers (Sprint, Verizon, and AT&T).
It is important to note that the total cost of a phone is not dependent on whether you choose prepaid or postpaid service. Either way, you
will end up paying for the retail price of the phone. The only difference really will be that you can do the 24-month EIP if you decide to go postpaid with a major carrier.
I hope I didn't confuse you too much. If you have any other questions, come back here and we'll try to answer them for you.