I asked my sons (14, 11, 8) what their choice would be, and all 3 picked Whispering Canyon Cafe. For them it wasn't a choice based on the food, but rather the uniqueness of Whispering Canyon Cafe. It is filled with all sorts of shenanigans and adventures. While I don't want to give away too many surprises, I'll give you a hint. When guests ask for ketchup, a cast member will announce to everyone the request. Then any table that has a bottle of ketchup will deliver it leaving the unsuspecting guest with many, many bottles. Needless to say, it is not a place for quiet dining.
If you'd rather have a quiet lunch and a chance to escape the sun, San Angel Inn will provide a lovely break in the day and is certainly convenient to get to if you are already in Epcot. When dining there, it is easy to believe you are indeed in enjoying an evening meal at a Mexican piaza.
Have you looked at the menus of these restaurants? www.allearsnet.com has all the menus of WDW restaurants. Just click on dining at the top menu to see the menus. Just thought that may help with your decision making, too.
Have a magical trip!