I love playing overstuffed households, at least for a while, although it's often a relief when I finally move out one branch of the family and the house becomes much more quiet and calm. But if you keep them both, you'll get to see how the kids of your boys and their husbands turn out. And you might just want to move on from one of the families before the other. I find sometimes that once some sims of mine have accomplished whatever goals I'd initially set for them, I'm a bit disconnected and ready to watch them from afar. Or, you could just say that playing with an overstuffed household, at least temporarily, lets you postpone your decision until you have all the necessary information, like, say, which grandchild is your favorite.
For the record, if I had to pick one, I'd go with Euan. I like his look better, and his name as well. Not that there's anything wrong with "Fife," but the name just doesn't seem to match the sim (no criticism intended, it's just all in my head). "Euan" evokes what I imagine that sim would be like if he were reading in his house's library as the daylight slowly faded. But like I said, it's all in my mind.