What features make a good fitness tracker for cyclists?
I come to this question as (in chronological order) a speed skater, cyclist, and runner. I have been a full-time Data Whore since early 2007. I have used various Garmin devices and smartphones. What I have found works best is the device that is on your person, with a sufficiently charged battery, and actually recording data.

I spent five years using a Garmin Forerunner 205 on a daily basis. That thing rocks, but Garmin’s device firmware that provides the user interface is just universally terrible. In spite of that, they are still very good devices for their intended purpose. After swearing I’d never buy another Garmin I was given a 405CX and after giving that away a few months later, was given a Forerunner 305. I still use that to this day.

Smartphones are great for “light” use, but when the GPS radio is running full-time, as required for an accurate athletic recording, battery life will absolutely be an issue. If the total riding time in a day will never exceed a couple hours, a smartphone could work. If devices such as heart rate monitors, cadence sensors, etc., are desired, they can be used with various smartphones by way of either ANT+ adapters or using Bluetooth accessories.

I’ve also long had issues with chest strap HRMs, in that they simply don’t stay up when I run. If I tighten the strap enough to keep it in place, it makes me nauseous. But recently I have realized that putting the sensor on the left side of the ribcage just below the armpit still gives a good read and stays in place better, and that even letting the whole thing slide down so that the sensor is over the solar plexus still seems to work. And I’m a relatively thick-chested male; conventional wisdom is that smaller females tend to have an easier time getting a read because there is less tissue to attenuate the electrical impulses.

Now, all that said… I say spring for a standalone GPS device if possible. They will offer significantly better battery life, and at least as of late 2014, still use much better radio hardware which makes for more accurate and less glitchy data. Many of them can be quick-release mounted on either handlebars or a wrist strap. But if you do go with a phone positioning accuracy will be close enough, and without a whole lot of biometric data (not just heart rate but also oxygen usage), caloric expenditure is just a wild guess anyway.