Having owned both for many years as daily drivers (300tdi for over a decade and a Td5 for a decade now) I can say, without hesitation, that the Td5 is as reliable if not more reliable than a 300. The Td5 is far more refined and smoother all round. It can be modified to provide almost double its factory HP output. The 300 Tdi has as many known issues as the Td5. primarily its all due to poor maintenance and or abused examples.
My 300 had a head gasket blow at 120,000, but it was expected. My Td5 has been flawless for 120,000 since I’ve owned it. It needed a started solenoid rebuild, and a water pump in that time. The 300 needed a head gasket, timing belt kit, vacuum pump, starter and a new turbo in the time I owned it.
Bottom line is know what you are getting into and be prepared to sort the motor you choose. A well sorted Td5 will easily run for 400,000 without major issues. The Td5 bad rep and poor reliability rep was due to its early run 10P motors and cracked cylinder heads in the injector casting areas as well as oil pump bolts which left the factory without loctite on the threads. These major failure issues were all mostly sorted under warranty. Lastly there is the perceived issue of electronic injection in the Td5 compared to mechanical injection on the 300 and it is bullet proof in my experience and not a justified supposed weak point.
Reality in my experience is that the Td5 is as simple and easy to work on as the 300 Tdi and easily as reliable. Go for a Td5... or don’t
Good luck
