Even if Sony is not the best or the fastest in updating its phones, it does take into consideration support for AOSP more than any other manufacturer. It usually contributes tons of code to Android and it encourages devs to work on their unlocked devices. Sony just announced AOSP support for Xperia E3 and Xperia T3, which means that most if not all Qualcomm-based phones made in 2014 will be running on pure Android soon.
This mopve is part of Sony’s open device project where they do not provide Android ROMs but they do release the hardware binaries for each device. These drivers help devs roll their own versions of Android and the ones we are talking about are destined to KitKat and Lollipop. Sony even gives out instructions on building Android from the AOSP.
The current supported devices are Xperia E3, Xperia T3, T2 Ultra, Z3, M2, Z3 Compact and others. In order to flash non-standard Android builds you have to unlock the bootloader which means the warranty will be void. So be careful!