1. Power (averaged over a useful time period; like 3 seconds and over an interval)

You want to gauge how hard you're riding so that you get the maximal benefit (strength, speed, stamina) from your workouts - to dig deeper and ride harder when that's possible, to ride at sustainable speeds instead of starting fast and blowing up so the second half of your ride is unproductive, to stop intervals when you can no longer sustain the power output.

Perceived effort is not very useful for gauging how hard to ride since it varies radically with how hard you've been working today, yesterday, and even six weeks ago.

200 Watts at a heart rate of 150 bpm can feel as easy as 120 Watts at a heart rate of 120 bpm.

Heart rate is better, but lags power output so it's easy to start off at an unsustainable pace. The lag also makes it useless for quantifying shorter anaerobic efforts. Heart rate also drifts, with influences from heat and dehydration.

Speed isn't a good indication of effort since slight false flats can make huge differences (up .3% can take 50% more power than down at 18MPH) and winds can have significant effects. Was your sprint faster because you were more powerful, or was there a slight tail wind?

2. Heart rate

Heart rate is better than perception or speed for pacing. Training with heart rate improved my speed a lot - it helped with pacing so an entire ride or interval could be as fast as it could and was a good reminder to dig deeper when I could or back off on recovery days.

Combined with power it gives you an idea of whether you're improving like you should (more power at the same heart rate) or it can be a tell tale for over-training (elevated heart rate for a given power when it otherwise shouldn't be high).


3. Recording at 1 second intervals with download to computer

You can pickup a lot of useful information from the recording. Sustainable power at various time durations (and where you need work) can be determined from any hard ride and not just intervals. You can look at the shape of your power output during planned intervals and see whether you're starting too hard or not giving it 100%.

With just heart rate you can still get time in zone and can monitor acute / chronic training load to help avoid over-doing it.

4. Distance

It's nice knowing where you are and how far you have left to go. Knowing that you only got 2000 miles out of your last rear tire might suggest buying a different brand.

5. Cadence

Getting enough speed in your legs is good - you won't make maximum power if you can't spin 120 RPM and a computer display is easier than counting when you're trying. You can identify problems (you get dropped when you spend over 5 minutes above your one-hour power/heart rate at less than 75 RPM). You can see what the effects are and adapt if there are benefits to pedaling faster or slower than your self-selected speed.

7. Backlight

It's nice for everything to still be visible when it gets dark out.

8. Speed

It's fun. Average speed will help predict arrival times.

9. GPS location

It's easier to match ride information to the download with a map instead of looking for that big hill as a period of high effort with low speed.