For me, this is less of an issue. When I was younger (Jr High/High School) I somehow ended up calling a lot of people "mom". (mostly the mom's of friends - was easier than remembering their names!)
So, I think I just sort of go back and forth. Sometimes I call her by her name, and sometimes I call her Mom. Whatever comes out. But, she does the same - sometimes refers to herself as mom and others by her name. She has also never requested that I call her one thing or another.
Hope you figure out something that will work for you both! Too bad there aren't more choices like for Dads.... If it were the FFIL I would suggest "Pops"! : )