In a few weeks' time, the iPhone 5S will be out and in the quarters ahead the iPhone 6 release date will take place, keeping the pressure on existing iPhone owners to make the decision - whether to upgrade or not.

Thankfully, crossing is made easier by iPhone trade-in deals that allow iPhone users to ditch their old units in order to enjoy the new model - for this year the 5S - and experience the so-called iOS 7 killer features and the inner component jumps.

The coming two weeks, according to Gotta Be Mobile, is the best period to decide for would-be 5S buyers to put their iPhones on the market because waiting too long will chip away considerable value off their handsets.

"There is a predictable 15-20 percent value decline seen across all older iPhone models in the six-week period surrounding the new iPhone launch," the GBM report quoted NextWorth.com as saying.

NextWorth is popular among iPhone trade-in deal seekers as it offers price lock-in period, meaning those wanting to swap their phones can provisionally 'close a deal' and get the best price possible for the next 30 days.

Within that time frame, users do have to ship their units to NextWorth, allowing them to use the handset until the iPhone 5S hits the store shelves.

Most analysts believe that the commercial debut of the 5S is set 10 days after its Sept 10 unveiling, giving sufficient window time for deal hunters should they plan to secure a NextWorth term by late next week or in the last days of August.

Similar offerings can also be found on Amazon, eBay and Gazelle but it the latter that matches the juicy trade-in packages listed in NextWorth, GBM said. Amazon, for instance, only dangles 7-day price lock-in period.

The same report also noted that the best deals are up for grabs for the early birds. That would mean iPhone owners willing to ship out their units before the 5S release date, and use a backup phone at the interim, will surely avail of the best value for their ageing phones.

The same applies for those eyeing the iPhone 6, which analysts said will not arrive until Q1 2014.

As usual, iPhones in perfect working condition minus the nicks and scuffs merit the highest value. For the iPhone 5, it is the 64GB variant and AT&T issued that attracts the best appraisal at $410. The Verizon release of the same kind will fetch $370.

The lowest that deal seekers can get is $314, which is for the 16GB iPhone 5.

The iPhone 4S remains a swap magnet with its value ranging from $171 to $245, depending on storage configuration and condition of the device. For the iPhone 4, the best price it can lure is $140 while the iPhone 3GS deal is limited to $33, tops.

As mentioned, the iPhone 5S is set to hog the limelight on Sept 10 while the actual availability will come by mid-September. As for the iPhone 6, release date is likely between January and March 2014, giving enough time for buyers to weigh the pros and cons of upgrading to the next iPhone level.