Most notable differenve is the larger screen. The charge 2 can display time, date, steps, and heart rate all on one screen. The charge HR only displays each at a time.
There are other additions to the charge 2 including showing when you're in fatburn and exercise heart rate ranges, as well as ability to set reminders to walk throughout the day and bedtime reminders. I am sure there are other differences, but those are what come to mind.
Also, I like the charger on the charge 2, the cord on the charge hr seems archaic in comparison.