On Wednesday, Sept. 12 Apple will unveil the next iPhone. When they do, millions will begin unloading their older handsets to get ready for the upgrade.

How much is your older handset worth? That depends on the vendor you choose. While you can certainly try your chances on Craigslist and elsewhere to get more money, we recommend using one of the following online services to sell your iPhone 4S/4 and iPhone 3GS: Apple, eBay, Gazelle, and Glyde.

Here is a look at what each service is offering for iPhones in “good” condition as of today, Sept. 5:

Please note: These prices are for those iPhones with normal wear and tear. Plus, all assume the iPhone is free of water damage, and comes with its original cables and AC adapter. You may get more or less than the price quoted.

iPhone Trade-In Prices, Sept. 5, 2012

iPhone Trade-In Prices, Sept. 5, 2012

Apple

Apple's Recycling Program

Apple’s Recycling Program

Apple is currently accepting 4S handsets as part of their Reuse and Recycle Program. For each trade-in, customers will receive an Apple gift card that is usable online and in Apple retail stores.

Pros: You’ll get an Apple gift card once your iOS device is processed.
Cons: Apple isn’t sending out electronic gift cards. In other words, you’ll have to wait for the mail to arrive to spend the cash.

eBay

eBay Instant Sale app

eBay Instant Sale app

The auction site is perhaps the easiest in which to sell your handset. To lock-in a purchase price, download the free eBay Instant Sale app and answer a few questions. Once your iPhone is received, you’ll receive PayPal cash.

Pros: Quick process to sell your handset.
Cons: Prices quoted aren’t always the best available.

Gazelle

Gazelle

Gazelle

One of the first “get cash for my device” solutions, Gazelle offers three ways to receive your cash: through check, PayPal, or through an Amazon.com Gift Card. If you choose Amazon, you’ll receive 5 percent more than the quoted price.

Pros: Great and reliable service that has been around for a long time.
Cons: Prices offered typically drop quickly as we get closer to the release date of new handsets.

Glyde

Glyde Upgrade Tool

Glyde Upgrade Tool

Unlike the other options listed, Glyde attempts to match buyers with sellers. In other words, if no one likes the price you set, you’re out of luck. Still, Glyde does suggest market prices that are much higher than those offered by other sites. Plus, the site’s upgrade tool is fantastic. You’ll receive your money directly in your bank account.

Pros: You set the price and get paid once the buyer receives the iPhone. Site is easy to maneuver.
Cons: It might take longer to find a buyer. Watch those seller fees.

As an added bonus, the first AppAdvice reader that uses Glyde to sell their iPhone will receive a $200 gift card to spend on items on the site. For more information, please contact me directly at bryan@appadvice.com.

Apple will unveil the sixth-generation iPhone on Sept. 12. It is expected to arrive in U.S. stores on Sept. 21.