T-Mobile is sure making a compelling case for folks to join its network with a new iPhone 5 this Friday. The carrier is offering competitive unlimited rate plans, contract-free 5th gen iPhones for $99 down and $20 per month, and it’s just sweetened the deal.

In a press release last night, T-Mobile announced a new trade-in offer for iPhone 5 buyers that could potentially eliminate their down payment and earn them up to a $120 credit, depending on the value of their device. We’ve got the full details after the break…

From T-Mobile’s press release:

“Delivering on its Un-carrier promise to change how wireless is done in America, T-Mobile USA, Inc. today announced it will supplement its amazing iPhone 5 upfront pricing with a special trade-in offer when the revolutionary smartphone arrives in T-Mobile stores this Friday, April 12.

“Our message to iPhone 4S and iPhone 4 customers is simple: bring in your device and trade up to iPhone 5 on T-Mobile,” said Mike Sievert, chief marketing officer for T-Mobile. “We’re making it incredibly attractive to buy an iPhone 5 by pairing an un-beatable upfront price and trade-in offer with Simple Choice, the most hassle-free and affordable rate plan in wireless.”

The trade-in offer will allow well-qualified customers who bring in their iPhone 4 or 4S to get an iPhone 5 for $0 down, but of course you’ll still have to pay the monthly payments. But depending on the trade-in value of the device, you may also receive some sort of credit.

It’s worth noting that T-mobile’s trade-in promotions appear to fall in line with what third party electronic resellers are offering for used iPhones. Gazelle, for example, says it will give you $150 for a 16GB iPhone 4 in good condition, and around $200 for an iPhone 4S.

For those interested, the deal begins Friday, when T-Mobile officially launches the iPhone 5, and is good through June 16. If you’re still on the fence, be sure to check out our post on the difference in cost of iPhone ownership between T-Mobile, Verizon, AT&T and Sprint.

T-Mobile is the last of the four major US carriers to officially offer the iPhone, and is the last major player to launch its LTE network. But it says that it has ambitious rollout plans, hoping to cover 100 million people with LTE by mid-year, and 200 million people by December.

So, who plans on snagging an iPhone from T-Mobile this weekend?