To me, "mutha" is a phonetic spelling and "mother" is what I'd call...well, "proper" spelling. In general, I don't really like phonetic spellings. frimfram brought up Spike's accent--how some people try to write like he talks. Because we don't have spellings for the way he says things, people often make things up, and sometimes the making things up makes it illegible. Even if it's not illegible, if you write well enough I will hear his voice and you don't need to spell it out for me.

But "mutha" is kinda different in that it's in common usage, everyone knows what it means--like "gonna". I think "gonna" is in the dictionary these days, but my guess is that's fairly recent. But everyone knows "gonna" means "going to", and for a long time it's been accepted in text.

Um, anyway, it's not like I have an answer. I guess with most of these things, it's a matter of personal preference. I prefer "mother" but I could see why others would see it differently.