I guess the first thing you have to decide is exactly what you want to do with the monitor. If you just want to use it to keep your training intensity within certain limits then you can get away with a fairly simple monitor that just has a stopwatch and a heart rate display. The simpler the better really, because you don't want to have to think too hard about which button to press when you are running, and you don't want an over-complicated display - the heart rate should be shown in large digits that can easily be read at a glance.
The next step up would be a monitor that had training zone alarms (i.e. an alarm that goes off when your HR goes outside of a max. and min. rate which you have set before the run). I found this feature useful when I first started to use my monitor, because it does take a bit of time to get used to running within set HR limits. However, I hardly ever use the alarms now - I can usually tell what my HR is without even looking at the monitor!
Then there are monitors that will record your HR at various intervals throughout the run. This is useful if you are running the same routes every week, and you can monitor your progress (or lack of it!) by entering the data into a spreadsheet and plotting the curves. There are some monitors that have an interface and software which allow you to download this data directly - but they are rather expensive. I think the recording feature is useful if you are training for a specific event and you want to monitor your progress carefully, however I rarely use it. The other use for the recording feature may be to record your resting HR when asleep to check for the presence of a bug if you suspect that you have caught one!
If you are planning to run in the evenings, some monitors are fitted with backlit displays. I have had no experience of these.
The other thing that has a large influence on the price is the precision of the monitor. Some monitors will only allow you to set training zone limits in 5 beat steps, others like mine allow single beat adjustment. Some are less accurate than others - I think the Polar monitors are claimed to be more accurate (accurate to +/- 1 bpm) than the Cardiosport (which I think is +/- 2 bpm), and they are definitely more expensive. But of course the training zones are calculated on a certain percentage of your max. HR (plus taking into account your resting HR), and if you use the age related maximum (which is good for about 95% of the population), then the training zone limits will be far less accurate than +/- 1 bpm. The only way to be sure of calculating the correct training zone limits is to undergo a maximum HR test, which I certainly do not recommend (anyone over the age of 35 should not do this). So I don't think the accuracy comes into the equation.
I have certainly had no problems with my Cardiosport Excel monitor and it was a good deal cheaper (at £85) than the equivalent Polar monitor (which was well over £100). My running has improved a great deal since I got the monitor, and touch wood, I have had no serious injuries since I got it. It's great for telling you just when you should take it easier (when you are prone to injury and illness), and when you can go for it! I never use mine in races, other than the fun run, because I consider it to be an unfair advantage especially on longer races like half marathons. Plus of course if you spent any time looking at the monitor in a cross-country you would soon find yourself on the floor!
I have a book at home, which I bought before I got my monitor, which I would recommend you read before going to buy a monitor. It will help you decide just what you want to do with your monitor, and which features you need and those which are unnecessary. It will also help you to cut through the jargon in the shop and ask questions before you buy. I can bring the book in for you to borrow if you like.
Kevin Smith, 3 July 1998