Geez, it's hard to say.
"True cinnamon" is species Cinnamomum verum. Cassia is the very closely related species Cinnamomum aromaticum. There are other species in the genus Cinnamomum that are marketed as the spice "cinnamon".
Cassia is alleged to be the more common (in the US market), but stronger and coarser spice. On the other hand, some of the non-verum "cinnamons" are the ones that are clearly native to Southeast Asian cuisines, so if one enjoys those dishes and wants to be authentic, you should use the "fake cinnamon".
I don't have enough experience at this level of detail to make a judgement. I'm sure I've had both true cinnamon and cassia and indeed, some of the other species, but I haven't found the differences to be on my culinary radar screen.