My understanding...
Little to choose between 200 and 300. Very similar power, although an engine transplanted from a Disco will make slightly more. 300 quieter than 200, although arguably more prone to going wrong (still rare though). Some 200 spares (cylinder head) getting harder to find.
TD5 quieter and more refined, however many more electrics to go wrong compared to 200/300...fact.
TBH I'd be buying a Defender on budget and condition and intended use. No point in going "oooo, it's a TD5 with electric windows" if you can poke your finger through the rear crossmember. If you are using it as a daily, you might prefer the refinement of the TD5. If you plan to do serious off roading/wading or go on an overland expedition to a less developed country, I'd be going for a 200/300 that you can fix with a hammer not a computer.
IMHO the Defender is not underpowered, it's under geared for normal road use unless you are regularly towing really heavy stuff. My own vehicle is a 200Tdi with a Disco transfer box which is the cheap and easy modification that makes it perfectly fine for normal road use at nornal traffic speeds.
Interestingly enough, I, erm, "compared" my 31 year old 110 200tdi (Disco engine) with my best mate's almost new Puma as we were following each other the other week and there was literally nothing in it.