I have both at the moment, only because I bought the 300tdi auto last year and it has been magnificent, even towing a huge twin axle Lunar caravan it has been returning 25 mpg, but that said they are a bit noisey compared to the TD5. I have taken the EGR off and the 300 really does fly. The TD5 does seem more refined and the 5 cylinder engine smoother and quieter, but it does not steam off the line as quick as the 300 but I have yet to lose the EGR. The seats in the TD5 I am informed by passengers are not as comfy as the 300 although as the driver I dispute that. The air suspension certainly makes a difference with ride quality but can also be a cause of problems due to holes in the suspension air bags or compressor woes. I still like the TD5 but am so pleased I have the 300 to fall back on especially as the auto box has now started playing up on the TD5 which pointed out something else which I find annoying, they do not fit a dipstick to the automatic box on the early TD5's (2001) I don't know about later ones, so it's either into the garage to get it checked or up on the ramps and diy. Oh and by the way, they all suffer from water leaks from the sunroofs.