I have and use both. As near as I can tell both are pretty accurate to me--within reason both devices have their strengths and weaknesses. And sometimes people use them in a way they are not intended. I have used both for about three years now.
The main benefit of the fitbit is that it is meant to track and encourage general activity throughout the day. There are other devices that do this too. I had read some articles about the health benefits of general activity in addition to exercise and in decreasing sedentary time. I was already working on that, but wanted a way to quantify my progress as I like seeing stats and numbers. I tried a pedometer, but just didn't like it (I tried a few cheapies). The well known 10,000 steps a day goal is really just a way to do this--though I don't believe there is really one ideal step count for everyone. I originally bought a fitbit for that and for general health tracking and a calorei burn estimate. THe reason I have stuck with it is it is so small noone sees it and I forget I am wearing it. With the stats it reports and the goals--I have a number of different options for activity goals--step count, activity "score", minutes spent in different activity levels, floors climbed, "calorie burn", distance, etc. So if I get bored with one type of goal I can focus on a different goal. All ahve the same result--encouraging me to keep moving throughout the day. I personally find the calorei burn estimate very in line with my actual results when I log food intake and pretty comparitve to what other sites (and Spark) estimates for me when taking into account exercise or general activity level.
I personally would not use a heart rate monitor for all day activity tracking--that isn't what it is designed for. I know some people do this, but it is an off label use and probably not accurate. I use my heart rate monitor for intentional exercise. It is really just designed for aerobic exercise. But I also use it to monitor which zones I am in, to time rests during intervals, to get me to push harder, and to see improvements in my fitness.
I don't think anyone needs to use either of these devices. People get fit all the time without them. But they are helpful and motivating to me and for a lot of other people.