Sooo, this topic has a huge potential (and temptation) for trolling but I'll try to keep my answer to factual facts
Form-factor:
The Troopy is longer than the Def110, it means slightly more length available inside, so longer bed or being able to have a little walking space available even when bed is made. And a little more storage space but I don't believe it's a worthy difference.
Longer also means longer wheelbase so worst turning radius, more difficult to park in cities, etc.
The Troopy has a high roof (125cm of internal height), the Def110 has a higher roof (~110cm I think) than almost all SUVs but not as much as the Troopy. This means that in both you can create a seating space in the back where you can be confortable to spend time and that (almost) no other SUVs will offer you. This for me is the real unique selling point of those 2 vehicles to build traveling/living vehicles. That being said the extra cm of the Troopy make a huge difference here, considering you want to let 85-90cm of head room to sit comfortably upright it means that your bench is at 25cm on a Def while you can push to 40cm on a Troopy (depends of how tall you are obviously) which means more storing space (and more usable for big items) under the bench and a better knee angle when seated.
That being said, if you plan a roof conversion the original roof height doesn't matter. Also if you plan a setup with no seated living space or to live on your roof (RTT...) the high roof isn't a big advantage anymore and means more difficult access to the roof and higher gravity center.
Troopy even without a roofrack is also just slightly taller than most parkings, height limit bars, etc. In Europe for a few cm it goes in the second category on toll roads, etc.
Troopy and Def110 are available in 3 doors van with flat rear. But only the Def110 is available is available in 5 doors.
5 doors is obviously a big advantage if you want to use the vehicle for some regular day to day usage but even for a traveling setup it's an advantage. 5-doors means a lot easier access to the storage (= better use of space) and also a non flat floor. Non flat floor can appears as a disadvantage to load cargo but if you are to build a setup I don't think it is, and the rear footwell allows some extra storage space, also allows to fit taller items (like a fridge) that would be higher that the bench setup otherwise.
A 5-doors Def110 could compare to a 76 but is clearly longer and with a higher roof, so actually allows setup that you can't really make in the 76..
Mechanically:
From the 90's and diesel variants, Toyota 70series heavy duty will have a big NA diesel while LR Defenders have a smaller turbo diesel (similar to light duty 70series), both producing a similar total power.
HD 70 series have a part time 4wd and leaf springs all around (before 99) while Def have an AWD transmission and coil springs.
So more rugged versus more modern / comfortable.
I would highly recommend this series of videos on the technical differences between world spec Defender and 70series:
) and tons of how-to Youtubes.