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.
I think it depends on what you are trying to achieve. If your doctor tells you (as mine did me) that you need an HRM, that means, Doc is concerned, you should be too, and so, accuracy is extremely important. However, if you are casually measuring HR and accuracy isn't important, or, you have other means to account for accuracy of a particular device, or, you can change the method of calculating HR, then you have lots of options.
Right now, there are two popular methods on the market to measure HR. One is optical heart rate monitoring, which uses LED to measure the expansion and contraction of capillaries, which do so because of the heart beats. The problem is that certain exercises can cause muscles and tendons to affect the contraction and expansion of capillaries, in addition to the action of heart rate, and so, OHRM method is not usually reliable when used during some workouts. Weightlifting, martial arts, combat fighting, some calisthenics, etc, anything which uses the arms, can garner an inaccurate reading. Walking, resting, some swimming, some yoga, etc which do not generally use the arms, can garner a more accurate reading.
OHRM is mostly found in wrist-worn devices, like FitBit. The latest news reports show these devices to be as low as 60% accurate, and cite the reasons I mention. Read the sources to find the details. These are convenient and comfortable devices, and as long as you know the limitations and can work around them, these are fine.
The other HRM technology are the chest strap-ons, these usually go around the chest and measure electrical impulses, much like an ECG machine. These are much more accurate, but many people find them to be uncomfortable, and the devices are devoid of the many other features a wrist-worn device has.
My guess is, if you find a device which uses chest strap-on for HRM combined with wrist-worn for data gathering and other features, that would be an ideal compromise when you need to measure HR during those exercises which are known to garner inaccurate readings.
Note that the BuzzFeed article, the last one, went on the cheap and compared their results with a chest-worn device, which they claim to be "scientifically more accurate". However, they do point out an important warning: that FitBit specifically states that their devices are not to be used for medical or scientific purposes. That means, Fitbits, et al, are for entertainment purposes only. You'd have to get a device which IS meant for such purposes... and gue$$ what?
My doctor told me the devices are great, but for accurate HR monitoring, short of lugging around an ECG on a tricked out gurney, a watch and two fingers is best, if not convenient. He wanted my HR within ranges, and so, accuracy for me was not important to me as it was for one of his other patients who had severe weight and heart issues. In that case, his patient had to wear devices which were needed for accuracy, and had to have insurance take care of the costs.
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).