Definitely a tough choice, but I am gravitating towards the Celine. I also contemplated these two bags for a while, but I ended up going with the Celine Box. Every time I went to try on a Chanel classic bag, I couldn't bring myself to pull the trigger. It's beautiful but just not for me.
I'm not sure if you have been able to try on the Maxi in a store, but it's big and heavy. It's not a bag you can easily wear crossbody, whereas you can with the Box. I also love the fact that Celine is understated; Chanel is much more recognizable. However, I think the Chanel has better resale value, if that is something you want to consider.