Personally speaking in ManUtd's situation, if it's 1 in and 1 out, they are getting a better overall player who is more well rounded in Young, but Nani is the more "explosive" player of the two and younger. If they keep both in a eutopian world that would be a ridiculous gain, and very well could happen, but if they lose Nani it's still an upgrade imo, but slightly.
The overall situation as it regards to us is, Young never really even seemed to consider us, so like Kenny says, if they don't want to come, we don't want them. Now for me and most of us, not true, however, with Downing seemingly to be the next best option and then Nzogbia, it represents a larger upgrade simply because Downing has developed into a fine player, even better than where most rated him early season and fits a position where we've had nothing at all for years. Not only that, but like Bill says, it's vital to get the very best and enhance Carroll's potential too, because I hate to say this, but fitness/injury aside, he's got to really fill his boots big time very quickly.
End of the day, it's a win for both clubs, but represents a much larger upgrade and value to Liverpool because it essentially impacts 2 positions with 1 player... and allows us to see what we've got in that 35M purchase..