We'll nothing is ever certain, but this definitely is: Apple will be holding a special event on September 10th, where we expect Apple to present two brand new iPhone models. Rumors are rampant but the consensus seems to says there will be a lower cost plastic-backed 5C as well as a high-end 5S which will be strictly similar to the iPhone 5.
The iPhone 5 wasn't revolutionary, however they a complete redesign from the 4S. However, we expect the 5S to have a very similar look as the 5 less of a redesign element. If you were hoping for a massive screen like the Samsung Galaxy S4 you're probably out of luck. All the rumors have suggested that the same 4" screen from the iPhone 5 will be used in the 5S. As for the hardware upgrades the A7 camera is expected to have a higher pixel count and and LED flash for night time photos. iOS 7 will be exclusive to the 5S but luckily Apple says eventually they will release support for it to work on 4, 4S & 5 models.
Should you upgrade? Well, if you're still using an iPhone 4 and are eligible for and upgrade at the discounted priced then its certainly worth considering. If you have to to fork over $600+ for the retail price then I'd say waiting until September 10th to see what the iPhone 5S really is going to offer. Until then, there is no shortage of rumors to keep you entertained!