We just ate at Garden Grill last week and really enjoyed it. The food was delicious, service was great, and characters were very attentive. The scenery is nice too.
WCC is hit or miss depending on the servers; last time we went our food was mediocre, our service was slow, and there were very very few antics at all. On top of that, unless you're already at the WL, getting there is a hassle.
Garden Grill has the huge benefit of being right in Epcot, and for lunch, you'll save a lot of time getting to and from the restaurant. I'd do Garden Grill without question. My kids thoroughly enjoyed it.
Garden Grill is now on our repeat list whereas WCC is not.