What are the most important points I should look for in a heart rate monitor? I am looking at only using it when I work out (so not a fitbit, etc for sleep). I want a hrm and not just a glorified pedometer. I want to make sure its accurate, can tell me total calories burned, and optionally uploads to computer and/or has a chest strap.
The main things to look for in a heart rate monitor for exercise workouts is its accuracy, readability, functions and ease of use.
Accuracy - Check your radial pulse and compare it to the heart rate monitor for accuracy. At rest you can check it for 60 seconds. During exercise you can check your pulse for 6 secs and multiply by 10. If you need a heart rate monitor for cardiac rehab, ask your healthcare practitioner for recommendations.
Readability - If your vision is a factor make sure that the display is large enough and clear enough for you to read it easily while exercising. A night light or backlight function can help readability especially in dim light.
Functions - As you mentioned you want estimated calories burned and the ability to transfer data to your computer. Other considerations are the ease of set up, ability to set target training zones, alarms or percent-of-max heart rate display, lap timers, water resistance, and the ability to change the battery by yourself vs having to send it back to the manufacturer for a new battery.
Ease of Use - Having more functions than necessary can complicate its use, so choose according to your goals. Consider the chest strap comfort and ease of application.
Price and warranty are also a consideration.
I think it comes down to preference as long as it takes your heart rate accurately. I prefer the HRMs that provide a strap that goes around your chest. Most machine will pick up on this so you won't always need your watch (unless you have timers set up on them).
You can test the watch by finding your pulse (in your dominant hand) on the thumb side of your forearm near your wrist. Feel your pulse and count. If you count for 15 seconds multiply by 4 and then look at the monitor and see if it is close! Counting to 60 will be more accurate (no need to multiply by anything since you've counted for a minute).