Still the Qualcomm SOC as it is the most energy-efficient (by a big margin) and reliable. Plus indeed the custom firmware support, which comes in handy after a few years. The Helio x25 is a tad faster, but you will barely notice a difference unless you game a lot.
/EDIT: I bought a Redmi 4 Pro for my mom a while back, which has the exact same specs as the Note variant and it works like a charm. Really recommended if you ask me! So you won't be disappointed by the Note 4 (Snapdragon) if you can get it for a nice price.