When are you going ?
December 5 to 11 this year
Epcot has more to see, so I'd skip HS. Of course, Toy Story Land just opened.... which I'm sure is fun for the younger kids.. But Epcot still gets my vote. It's got a nice mix of rides & cool scenery in the future world area, The aquarium is also great. And the country pavilions in the world showcase are colorful and fun to wander thru.
HS is quite a bit smaller, and some of the rides & shows are aimed for older kids & adults (like TOT, RR, Indiana Jones). They're both fun parks of course.
I absolutely love Epcot. My second favorite park after MK.
Do the kids like Frozen? You could visit Anna and Elsa in Epcot’s Norway section.
We love Epcot during Flower and Garden Festival.
That said, with new TSL at HS, you might do that instead.
You would want to secure FPs though. The lines will be nuts there for a while.
HS might be more manageable size wise.
You will be going at Christmas time so Epcot will have the Epcot® International Festival of the Holidays going on with story tellers and food booths. However DHS looks pretty cool at night with the holiday decorations. Also if you were looking a day with a little more down time, I think DHS provides that as it shouldn't take as long to do the park.