Get the Mercedes.
From someone who owns both, you'll love the Mercedes much more. The engine will run forever, the car will last forever (perhaps unlike the TDI...made in Mexico..), and you'll continue your joy of owning a quality Mercedes.
The E300 will whoop the TDI's a$$ off the line, at the top end, and everywhere in between. I do like my Jetta's manual transmission, it's great for blasting around town, but the TDI's low power (let's all admit it) somewhat spoils the fun. The TDI has a great VNT turbo that spools immediately, unlike the E300 which will have some lag.
The interiors of the two cars, while similar in quality, are worlds apart. The TDI will make you feel like $50k, whereas the E300 will make you feel like $500k. The wood, the quality of the seats, trim, and switches, the overall spaciousness...they all add up. The E300's Bose system is much better than the TDI's Monsoon system.
Fuel economy. If your TDI is a stickshift, you'll see 50 mpg. My E300, driven somewhat aggressively, achieves 32-34 mpg. The way I look at it, on the highway my E300 is still getting better mileage than pretty much everything out there--save the TDIs and Priusucks.
I think the E300's lower fuel economy is worth the rewards of owning a Mercedes.
In my years of experience with the OM606, it is extremely reliable and easy to work on. Granted, the intake on a turbo model is a PITA to remove, but removing it becomes a mastered skill. Glow plugs require some patience (and luck) to avoid breaking, which I have experienced once. Fuel leaks can be common, but are easily tracked down and fixed. My one piece of advice upon becoming an E300 owner is to buy a battery charger--purging the fuel lines requires the engine to be cranked.
I'm not trying to put down the TDI--it's an excellent car--I just think that if you're going from a Mercedes to a TDI, you'll feel somewhat let down. The E300 will make you feel so much better.
Now, if your girlfriend gets the SD while you're buying yourself a nice speedy E300....there may be some problems there
