I have both a TD5 and a 300.
The TD5 engine is miles apart from the 300, smoother more powerfull, less agricultural and better to drive. If your intended use is more 'road' biased then go for the TD5, if it more 'off road' biased then go for the 300.
this is not saying that the TD5 is any worse off road, it's not, in fact it's just as good if not better (with Traction control), just it's electrickery can be prone to problems when you need it least!!! and there are more 'off road toys' available for the 300 series.
The cabin in the TD5 is a more pleasant place to be: quieter, air con, nicer controls, better lighting in the boot, better stock audio system and comfortable headrests.
Being all computer controlled the TD5 engine can be chipped... now your talking about a vast difference... a genuine sporty character!