I've got the flip out screen on my ancient G5 it's handy for shooting over the top of people, getting down low or shooting yourself, other than that the view finder / live view is all you need.
in terms of the two, it's a touch choice the nikon has a better zoom and low light so depends what you want from it.
If you can increase the budget what about a four thirds system, like the lumix?
In terms of portability you might also want to consider the superzoom (20x) travel compacts, all of these now have manual control and pretty good handling, plus you'd be able to get waterproof housings for them if you want to experiment underwater or extreme conditions.
On the flip side, what about a cheap (plastic body) entry level canon (as you've already got the lenses) this would be slightly bigger than a bridge but they are so light; 1000D 502g, G12 402g, Nikon 7000 310g as a guide a king size mars is just under a 100g and you can get a 1000D for less than the G12
Personally if I was looking for something portable I'd be looking at a plastic body DSLR or four thirds as I wouldn't want to be in a situation where I was thinking "If only I had my main camera"
A last alternative is get some light / portable lenses and decent camera bag for the 50D, I've taken my D300 skiing, round eastern Europe, mountain biking and walking with a decent bag you don't notice the weight as much and you've got all the features you want.