[LeighS]

I would say both could be used, depending on the circumstances surrounding the statement.

I will tell her if I see her
"Have you seen Sue? I need to speak to her"
"No, I haven't, but I'll tell her if I see her"

(I've just decided to tell her sth)

I'm going to tell her if I see her
"I haven't seen Jenny in ages!"
"Neither have I. I heard she's coming to the party tonight though"
"Great! Does she know Dan and Jill are getting married?"
"No, she doesn't . I'm going to tell her if I see her"

(I decided a while back to tell her sth)