Verified answer

Whoever gets Brees and Westbrook by far gets the better end.

It's probably pretty close, but I think I'd take Manning and Horn. Manning is a consensus top-3 pick in keeper leagues because of his year-to-year consistency, and trading Westbrook wouldn't be a huge hit because you get to replace him with Adrian "All Day" Peterson. What a steal in the 6th! From a pure value point of view, Brees and Westbrook are probably better, but this trade will improve your team.

Counter-offer to get a stronger receiver, or to give up a weaker back. You can afford to lose Westbrook with your roster (your late round backs are unproven but very good gambles--at least one of them will pay off), but you aren't getting enough value for him in that trade. If he won't give you his best receiver, he doesn't get your best running back. Manning is only a little better than Brees fantasy-wise, not so much better that you should take so little for Westbrook.