Mom or Mum?

I have been asked by a few people why I'm Petit Mom not Petit Mum. No it's not a typo or a misunderstanding of the English language. I understand in the UK we spell it as "Mum" but since I could remember I've always called my Mum... Mom.

My mom didn't mind until I got older, she would correct me every time I put "mom" down instead of "mum". This didn't stop me! To this day I still put in her birthday cards "To Mom" and on her presents it's "To Mom" and emails... your get the idea. I think after I left home and still continued to use this spelling she simply gave up as I'm older and able to get on with my own life. But should I use this spelling on my own son? Well to be honest I don't care if he calls me Mum, Mummy, Mom, Mama or the likes. As long as it's not a nasty name I'm happy with whatever he picks! If he's comfortable with calling me the name he has chosen then why should I argue? Mom may be the American spelling, but it's still the same word. However if he starts forgetting letters in words (ie: color and not colour) I'll probably tell him "that isn't how it is spelt" or something along those lines.