The TD4 suffers all sorts of problems - injectors, HP pump leaks, lift pump failures, sensor problems, pulleys delaminating, cranks snapping etc etc. Lots of engine related problems are a nightmare and expensive to resolve.
The 1.8 does suffer HGF - but that is the only problem you see going through the forum. Admittedly its a biggie, but they don't sound to complex to fix and are not expensive - so long as you address it as soon as any problems show up and you don't cook the head. They are also belt driven, so every other HG, you replace the belt/pulley at the same time
The 1.8 is only available in manual. The PG1 isn't the best box in the world, but its OK and pretty reliable and the clutch is cheap to replace. The auto in TD4 can suffer reliability issues and the manual, while the Getrag box appears reliable, has a very expensive clutch setup with slave hidden away inside the bell housing.
When you add in the cost of more expensive fuel and licensing costs - the 1.8 is probably as economical to run with similar on-road power - actually may be better on both counts. Its not as torquey though for towing and off tarmac.
The 1.8s are cheaper to buy, so overall, if you want a manual are probably a much better bet than a TD4.
But, not as good as an L Series diesel of course
