Yeah, I think an HRM would help you, though you will have to devote some effort toward identifying your max HR for the numbers to be meaningful. And then you probably want to do some reading on how train with heartrate. I will readily admit that I don’t use this data nearly as much as I could. I like the calorie calculations it provides and use it on all-day rides to make sure I am not working too hard, use it on rest days to make sure I am taking it easy, etc. In find HR too sluggish (reactive) to really use for short intervals, but it is nice for racing etc. since I know what sort of effort I can maintain, so can try to keep numbers within that range.
I think you want a bike computer that is mounted to the bike. I know some people do attach the watches to the bars, but the display seems really small. I just think if you are really pushing hard on an interval you don’t want to take your hand off the bars to check your HR and you want a device that has an easy-to-read screen. So I would vote for the 500 between the two, but obviously if you want to also use it for running then the Forerunner is probably the better choice. You might want to consider power-meter compatibility too; apparently data is a slippery slope …