Ali, when buying any kind of gadget consider what you will be using it for in three years time not just now, that will prevent having to upgrade watches when it's no longer fit for purpose because you've decided to do something longer or harder or different.
Then write a list of everything you want it to do.
A HR monitor is useful but only if you know what it's telling you. Don't get one just because you think you should or if you can't be arsed to do a Max HR test or read up about training zones etc.
A GPS system will do more of your mileage/pace needs without the extra need for a cycle computers. Some GPS systems come with bike attachments so you can get cadence monitors and such likes.
But really, start with the list and then compare with the specs.
Garmin is 'generally' the most popular manufacturer. In my experience they have realy good customer service, good software updates, excellent web site facilities to record and log training.
Look at the 310 if you plan to do tri (you can use it in the water) or a 305/405/210 if you don't plan to swim with it.