IIRC (from chosson classes) it's not a question of better or worse, it's a question of whether you fulfill the halachic requirement to keep a calendar if it's not in writing.
My teacher said that the drawback to an app was someone not learning how to keep a calendar. But if you learn how to do it, he had no issue with people using the app. Having said that, My Mikvah Calendar has the following disclaimer:
Although My Mikvah Calendar makes all calculations necessary for computing your onot of separation, it is imperative to still do these calculations yourself.
The online calendar may be used as a tool for learning how to keep a calendar, as well as double checking your results. This is because there are halachic opinions that say that the obligation to record onot of separation cannot be fulfilled by a computer
I still maintain my OP in response to saying an app will cause errors