It depends on what you are looking to track. I own both a Fitbit (2012 model, not sure of the exact name) and a Polar FT4 heart rate monitor.
I prefer my Polar FT4 because I like to do other things than just walk and climb stairs. I can't workout without it now. It tracks your calories burned really well.
I would consider the Fitbit more suitable for someone who isn't a huge gym goer and uses walking as their main source of exercise.