The regular Fitbit flex can fall off easily...I know a couple people who have lost them. The plastic wristband isn't very good considering the tracker is semi pricey. I can't speak to the charge HR but it seems to clasp like a watch instead, so that might be an upgrade.
For the Flex, if you only use it as a pedometer as most people do, it is a really expensive pedometer. It also has an alarm, tells you how "active" you've been (when your step count increases at a certain pace over at least a 10 minute period of time), tracks sleep patterns, and the app helps you track calorie intake and some other stuff. Nice features overall, but they're really not necessary given that you can use a clock alarm and use other apps for calorie counting (you can also use other apps paired with your Fitbit). I don't know how it can tell how many calories I've burned in a day, because if I check mid-morning when I wake up and don't have my Fitbit on, it still claims I've burned a decent chunk of calories. :shrug:
I don't know the cost of competing heart rate products, but if that's worth the cost to you then it makes sense. Just remember that with fitbit products you need to be moving your arms when you walk to get accurate step counts (if that's part of your goal). It won't get every step if you're walking but not swinging your arms (i.e. pushing a stroller, per their FAQ). Also for the sleep tracking, since it automatically tracks that with the one you're looking at, all they do (from what I can tell) is track movement during sleep. If you're wide awake but sitting still, it thinks you're sleeping. If you're sleeping but thrashing violently, it thinks you're awake. It's fun for a little while to see when you woke up at night, but it's just a nice-to-have.
I'm not saying Fitbits are bad products -- they're fine for what they do. They just charge a nice premium for putting together a few things into a more convenient package.
I found this review interesting -->
http://www.theguardian.com/technology/2015/feb/13/fitbit-charge-hr-review-heart-rate-tracker