One area where the 300TDi is definitely more reliable than the TD5 is the front driveshaft.

On the TD5, the front driveshaft has a double cardan joint that wears out easily and cannot be lubricated. When it wears out, it has a nasty habit of simply letting go, and that's when the real damage occurs.

On a 300TDi, the front driveshaft is a greasable uni joint.

Of course, there are solutions to every problem.... and the double cardan joint problem can be addressed. But technically, it's less reliable.....