OK, I'll have a bit of a go replying to this now, although it won't be as in-depth as I'd like to be! And before I really get going, I've only read half of The Stand, so I'll only be able to comment on that bit!
I agree that of all King's books, these two probably do seem to have the most in common with one another. Your reasons for their being similar are true, but it also seems a little vague, if I may be so bold. Quite frankly, I think any post-apocalyptic story would take a similar vein -- the thing about apocalyptic happenings is that lots of people are likely to die, and not just during the event but afterwards as well, as people try to cope with such a upheaval of their very way of life.
As to the groups going off into good and bad -- well, in The Stand you have Flagg and Mother Abagail to thank for that, with each 'calling', as it were, those suited to either side. Again, I think this would be something that would happen during such large scale events (not the calling, of course, but the grouping). You would have people who would probably group together, trying to salvage what they can, help one another, etc, and then they'd be those that would use the chance to rob, steal and generally take advantage of and revel in the the chaos already caused. As for Cell, the groupings are different because the bad guys are different. They group because they act like a flock, a form of telepathy bringing them together. And the good guys group together because, well, probably, 1.) so they're not alone, 2.) in order to have a better chance against the phone crazies, and 3.) in order to do their own flocking towards a place away from the Pulse.
As I've said, I haven't read The Stand in its entirety, so I don't know how it ends, but it's up for deliberation whether Cell actually does end happily for all. For a start, you can hardly believe that they will live "happily ever after" after such a large scale catastrophe has happened; life has been disrupted like never seen before, many people are dead, even more have had their brains effectively wiped. And on a smaller scale, there's Clay and his son. We can have hope that his son will be "cured", as it were...but there's always the chance that he won't.
And just time for a final note -- I really enjoyed Cell. I think it was a fantastic return to King's rather gory, action-packed and generally very entertaining writing. They're wasn't anything particularly deep in this novel, but a group of people striving to survive a harrowing event. Rolled along at a great pace, I loved the phone crazies, and overall was a great read for me, at least.
Those are just some of my thoughts, anyway! One day I'll finish The Stand. I will.