Both the mini and full-size One include HTC's 4-megapixel UltraPixel camera, which is designed to excel in low-light situations. You get the same basic sensor, and the same HTC software package, but the mini version of this rear-facing camera lacks optical image stabilization, or OIS.
The lack of OIS could potentially degrade image quality in low-light situations, but nothing about the mini's image caputure, in dark conditions or otherwise, suggested the camera was underperforming relative to the full-size One. That said, neither camera will save the day in truly dim environments. It's a phone camera, not a miracle worker, but that's okay with me. As long as it can shoot Instagram photos and corgi videos, that's all the world really needs.
HTC also ditched NFC and IR blaster support in the mini version of the One. I haven't used NFC on a smartphone since the time I set up a Nexus Q, and I can't imagine why anyone would ever want to use a smartphone as a remote control. Still, more and more peripherals (think headsets and printers) depend on NFC for pairing, so just be aware that the mini can't accomodate these devices—let alone any mobile payment system that depends on NFC.
Unfortunately, HTC still includes Blinkfeed in its Sense 5 interface wrapper, so you're stuck with HTC's vision of social/news aggregation whether you like it or not. The mini does, however, come with the 4.2.2 version of Android, while the original One is still saddled with 4.1.2. This version, along with a slightly updated version of Sense 5, grants you the awesome Quick Settings menu that's directly accessible via the notifications shade. It's a feature that makes me jealous every time I pull down the shade on my full-size One.
The HTC One mini is a respectable phone when compared against the broad continuum of what's available today, from freebie smartphones to $200 (on contract) elite models. But it's not a great phone—it's just a pretty piece of hardware that fits nicely in the hand, but features a smallish screen in an age when any display smaller than 4.5 inches feels too small.
HTC cut a lot of corners to hit its $100 price tag, and while it's nice to see build quality remains excellent, I can't recommend just 11GB of usuable storage and underpowered silicon when just another C-note can elevate you to current state-of-the-art performance.
Buy the mini because you love HTC's Sense UI, or the company's unique take on hardware design. Or because you just want a solid but not superlative $100 phone that's not the iPhone 5c. Beyond these benefits, the mini doesn't have much to offer.
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