"...With the heavy demand for iPhone 6 and iPhone 6 Plus batteries, Apple has probably run through much of its remaining inventory of those phones. Restarting production to meet demand for replacement units and batteries might take some time.
Keep in mind that if your iPhone is getting a batter replacement, that alone doesn't qualify you for a whole new replacement unit. To be eligible for a whole device replacement, your handset needs to have suffered damage to its motherboard, Lightning connector, or a component not easily replaceable. And even if you have been given the green light to turn in your Apple iPhone 6 Plus for a replacement, it doesn't guarantee that you will be receiving an iPhone 6s Plus instead.
There are major difference between the iPhone 6s and iPhone 6s Plus. The older model is powered by the Apple A8 with a 64-bit dual-core CPU running at a clock speed of 1.4GHz and includes the PowerVR GX6450 GPU. The iPhone 6s Plus features the A9 SoC with a 64-bit dual-core 1.84GHz CPU, and the PowerVR GT7600 GPU. In addition, the iPhone 6s Plus doubles the amount of RAM found in the older model, to 2GB. And the camera on that unit comes with a nice upgrade to 12MP on back and 5MP in front from the prior year's 8MP and 1.5MP, respectively."