Apple's release of its latest flagship smartphones, the iPhone 6 and iPhone 6 Plus, has been met with great demand.

Apple enthusiasts that want to be among the first to own the devices have lined up for the official release of the smartphones in Apple stores, if they have not yet already pre-ordered their iPhones online.

While the supply for the iPhone 6 and iPhone 6 Plus are currently outweighing demand for the devices, it is not stopping consumers from wanting to get their hands on Apple's new smartphones.

Consumers, of course, are always on the lookout to find which carrier has the best deal when it comes to obtaining an iPhone 6 and iPhone 6 Plus.

However, before comparing carriers, there are some things that consumers need to keep in mind.

First, with the supply shortage on pre-orders already backed up until October, the surest way to quickly get an iPhone 6 or iPhone 6 Plus is through carriers that already have the devices in stock. 

Customers that are eligible for upgrades with their respective carriers can take advantage of trade-in promos that will pose significant savings to the customer.

Carriers have different plans with different advantages, and it is up to the consumers to discern which one works best for them.

For AT&T, customers who sent in pre-orders will have their devices shipped to their billing addresses. Customers signing up for a new application can get the iPhone 6 at no cash out with a $100 bill credit, and until Sept. 30 customers can trade in their older iPhones as old as the iPhone 4 for a $200 value, except for the iPhone 5S, which has a $300 value.

For Verizon, customers can also trade in their older iPhones at the same values as AT&T for a 16GB iPhone 6 with a two-year contract. The trade-in promo is also until Sept. 30. Verizon customers that are eligible for the upgrade can take the promotion immediately to instantly get their new iPhone.

For T-Mobile, the company's promise is that it will match any offer made by any other carrier. T-Mobile will pay customers any difference between its offer and the offers of others, and for a limited time, it will also add $50 to the trade-in value. However, the difference for the company is that it does not subsidize the cost of phones the way the other carriers do.

For Sprint, in addition to the seemingly standard trade-in values, the company is also offering the "iPhone for Life" plan. Customers availing of the plan will not have any cash out for the new iPhone 6, but will be charged a monthly lease of $20 and a $50 monthly service fee. After 24 months, customers will give back the phone to Sprint to get the latest iPhone model at that time.

The iPhone 6 and iPhone 6 Plus are also available at retail stores such as Walmart and Sam's Club, which are selling the iPhone 6 for below $180 and the iPhone 6 Plus for below $280.