IMO - If if its not broken don't bother fixing it. Although, I have owned a Rena heater, I would just monitor the temperature closely. Maybe even make notes as to when you checked the temperature and what it was, to make sure it is holding the temp steady. IME, that has been the biggest issue with heaters is that the temp swings too much. IMO - 2 degrees is too much.
I am big propponent of Rena filters. But, if you need a new heater, IMO there really isn't a reason to get anything but a Ebo-Jagger heater. I have never had a single problem with one and I even forget to check the temps in my Ebu heated tanks. When I did, the tanks always stayed right on temp. The thermastats seem to be finest I have used. Soon, all my tanks will have only Ebu heaters.