"I need to make an appointment to see the dentist, but I've been putting it off."
I'm pretty sure the above sentence is grammatically correct, but does it sound more natural (particularly in American English) than saying:
"I need to make an appointment to see the dentist, but I keep putting it off."
"I need to make an appointment to see the dentist, but I've kept putting it off."
Does the use of the verb keep make the sentence sound more formal?