I like Marvin's overall versatility and size. He would be a great 3/4 off the bench. His contract has 3 more years (after this one) for around 23 million, so he isn't cheap, but I think he would help the Celtics a lot this year.
JO for Marvin works on trade machine and would save the Hawks a bunch of money long term so they very well might take that offer. Given JO's uncertainty this year, I'd rather have the safer option. Though it would likely mean Davis is gone at the end of the year, so perhaps the C's could then look to move him somewhere else.