Le Cellier is very good and it will be a little less expensive. It also has the advantage of being in the theme park, so you can schedule it for the night you see Illuminations (or schedule a late lunch and then go over for Fantasmic). (Did you know you can walk from MGM to Epcot - and end up not far from Canada?)
I haven't eaten at Artist's Point, my husband and father have. He (the DH) says the food is a little better at Artist's Point (but a little more expensive) and the atmosphere a little nicer. It has the advantage of being in your hotel, so you can choose a late dinner or schedule on a day when you want to go to the pool....
So my suggestion would be to look at your schedule and try and figure out which end of WDW you want to be at. Both restaurants are good and I would expect to be disappointed by either.
