This is the beauty and the danger of asking a question such as yours, Razor. Somtimes I'd get the feeling that you will get as many different answers as the number of people you ask. The good thing though is that there is a LOT of expertise to be found here and that is what makes this site the best of its kind.
Back to the point though (and here is yet one more opinion to add to the discussion), I'm a big fan of the A14/ 15 conversion. The upgrade doesn't require major modifications and you don't need to be a Mechanical Engineer in order to complete the job. Once fitted, it will retain a degree of originality and simplicity to work on. On the flipside, a CA18DET gives you injection, Twin Cams/ Turbo and and modern technology but requires more work in the conversion (plus a brake upgrade, gearbox change, etc).
Hopefully this will help you - If you want power and are prepared to pay for it, go the CA18DET and have a ball. If you want mild power, a simple conversion and something easily repairable, grab an A14/15 and put the cost difference into something else on the car.
But whatever you do, remember there are plenty of us here to help you with advice...
Best of Luck and Keep us posted...