Hey Rob,
I don't think there's much between the last two kits - they're both pre-painted and weathered and I think the extra parts count is because it has the 5 landing gears set-up as opposed to the 3 of the 'original'. Also the newer radar dish.
If it was my 12 year old, I'd go for for the 'Force Awakens' kit as it's the newer movie (and he's built the First Order Tie as well).
I've seen both of these Falcons built up....(well, snapped together) and they look good for what you get. With a little panel line wash and more weathering, they can look fantastic.
I'm sure he'll have heaps of fun, whichever one you get him. Make sure to submit some pics to the gallery here - always good to see the younger modellers getting involved in the hobby!
Hope that helps.
All the best,
Dermot