I don't understand how the left side of Betta would be the better one in general, tho . Maybe some particular Betta looks better from one side than the other, but I can't imagine any universal "law" about the left side being better.
Its just something i heard (or read) from the pros. They always say try to take one from the left. It just seems better most of the time. No personal experience in this. But lots in reading what others have posted. Have a go and take both sides. then we'll compare.