Interesting question.

In general I like to keep my rucksack as light as possible, the less weight on your saddle the better, if your rear starts to ache you're not going to go fast.

I would probably go for the slightly heavier bike, as long as the extra weight is attached low down and to something that doesn't rotate.  Front mech band will do.

If you added 5lbs to a tyre is would make a huge (and horrible) difference.

It would probably depend a bit on how heavy the pack and bike were in the first place, of course Fill your Camelbak with water and that's nearly 5lb right there.