I use both and love them. I didn't aquire both at the same time though, I am not sure whether I would get both at the same time just because of the expense and that there is a bit of a learning curve in learning to make the most of both. But they can work well together.
First I want to say, that the benefits I found using both could quite possibly be achieved with no device. Improving health and fitness is really about doing the work--eating healthy, being active, sleeping enough, drinking enough water, etc. We can do those things without any gadgets or tools. If you like data, measures, gadgets and tools either of these devices can be helpful and motivating. More of a nice to have than a must have.
Which one if I had to choose one? It depends:
*My fitbit gives me a record and archive of my overall activity level, sleep and a few other health measures that I choose to log. It breaks my day up into minutes spent at different activity levels and I find that pretty accurate for me with a little adjusting on my part. Also, I enjoy working towards the goal of 10,000 steps and 10 floors a day. It helps me make sure I am active in addition to intentional exercise. It does give a calorie burn estimate for the day that feels pretty accurate to me. It is just an estimate though and is pretty much in line with Sparks on most days. It is small and discreet so I can wear it clipped to my bra and no one sees it. I will say some of what the fitbit does for me could probably be done with a good, small digital pedometer.
*My hrm, I do use it for a personalized calorie burn estimate but I also take it with a grain of salt. The estimate it gives me is lower than what Spark or any other online database gives me so it is a more conservative estimate for me. Also, it is useful for seeing improvements in my fitness level as the same workout becomes easier over time and results in a lower heart rate and lower calorie burn. Also, when I intend on a vigorous cardio workout or am doing intervals it motivates me to work harder and helps me notice my recovery time. I do think some of the benefits can be achieved with perceived exertion for free, but for me using an hrm for a couple years now has actually helped me tune more into my perceived exertion. I do a variety of types of cardio including some less traditional, and it helps me compare intensities among different workouts and feel confident in my choices. In the past I've had doctors and other professionals tell me that certain things I do (like dance) don't have any fitness benefits besides stretching and try to encourage me to use gym cardio machines to get a real cardio workout. I personally don't enjoy gym cardio machines, but knowing what heart rates I achieve doing activities I enjoy have helped me feel more confident in my choices and defend them when I need to.