Regular stats are just what they say, the pure stats you have at that given time after all boost are applied(Mood, Group Skills, Awakenings etc)

Growth stats are a constant which is part of the formula that determines how much you gain in a specific stat with every level up/power up. I don't know the exact formula(doubt anyone but the devs do) but in general the higher the Growth for one stat the more it'll go up after you grow that ninja

Depends on if you wanna equalize them or not. Generally you wanna build on their strengths rather than have *l around average team and truthfully magatamas are the least of your worries, you can try them once on one of them and one more on the other and see what is best. It's things like Moods and such that the growths can play a defining role in. If for example you have a ninja with 600 Ninjutsu growth and 1200 Attack growth which only has mixed attacks and chases you get a bigger increase with a +nin mood than with a +atk