Specs for parts on Giant Defy 4 (2013) to decide on upgrades to transfer parts onto another bike I have a 2013 Giant Defy 4 and a new Montague Urban(UK)/Crosstown(USA) folding 21" 700c.

I'd like to make the Montague better for the occasional uphill commute home I do when I don't have access to a boot and a passenger seat (hence the folding bike) but I don't want it to be too appealing to thieves (however I don't mind making things appear cheap with mud and sandpaper!). It's built as an urban bike so the seat is comfort and the riser bars are for an upright position (so it's not great for a bent over wind resistant riding position). It only comes 7 speed with no front mech. Making the bike lighter would be lovely when I'm carrying myself and my gym stuff home, cursing at the geographically almost-guaranteed head-wind.

To that end I want to fit parts that would improve my ride (drop bars, better seat and anything else), but I'd rather buy good stuff for my Giant and then replace the not very good Montague stuff with the now surplus Giant stuff. (That's assuming the Giant hardware is indeed better.)

I can't find any useful info on things like the bars, which just say "Giant Sport 26.0" and which I can't find these on the Giant site. Roughly, the Giant is 10.7 kg and the Montague is 12.3 kg. Plus, without (what I consider to be) a reliable pannier rack option I carry the weight on my back. I'm fairly heavy and my road surface is fairly rubbish, I think I'm right to make weight loss a target for the bike ;)

Short of weighing the deconstructed bikes for parts, how do I proceed with this project? 

A couple of things to note: I've already made the Montague look like crap by using permanent marker on the text, muddy water and strategically placed plastic carrier bags so it won't be too enticing to thieves - new fancy kit on it would make it a target; also I've got a couple of non-stock parts on my Giant: Shimano R540 SPD SL pedals and a Nukeproof Plasma Speed saddle.

Replacing a groupset in step purchases whilst this is a good post and will help if I come to adding a front mech or increasing the cassette, I am not primarily interested in replacing the gears - 7 speed is OK for the most part - I would probably only change it if the bars, stem and seat and drop levers if I change to drop bars. I'm more interested in making as big weight savings as possible and getting a more comfortable setup for reducing wind resistance etc. Though I'm sure once that's done I'll be wanting different gears!
Really, the only thing you can do is investigate each individual part as you go along. 

With drivetrain parts you'll have to investigate how well they work with other drivetrain parts. E.g., if you change the derailleur, does it work with the shifters? Is the cassette compatible with the derailleurs? Etc. Try searching the site for stuff like drivetrain compatibility, shifter compatibility, and derailleur compatibility. The venerable Sheldon Brown is also a magnificent source for determining how things work together.

With fairly static parts like your handlebars and seatpost, it will likely come down to simply measuring to see if they match. You'll probably need to do so with calipers as these things can sometimes vary by a millimeter, or perhaps even less. However, Batman mentions in the comments, you may run into comfort issues. Whether these issues are within your range of tolerance will depend on your bike's setup, your body proportions, flexibility, and the individual parts that you're swapping.