Update speed
The main advantage to me is getting updates on a sane schedule, instead of having to wait indefinitely or just being flat-out denied updates because the manufacturer's attention has moved to newer models.
Personally, I wish they'd move to a model more like the PC market: LG and co sell hardware, Google and co supply software to run on it. You don't get your Windows or Linux updates from Dell or HP, they come straight from MS/Ubuntu/whoever.
I'm quite interested in the new WileyFox handsets, the Swift and particularly the Storm: dual-sim support, SD card (though irritatingly, not both at the same time: at least one handset does support both, but not this one) and decent screen, camera and CPU. Pretty much everything I'd ask for in a handset, for £199! I just hope they stick to vanilla Android, so it can be kept updated easily for the long term.