I might be biased as i have a 2012 X3 20D, but that would be my choice no doubt.
Firstly resale value - seems the X3 35's go for way less than a 20D of the same year and mileage and spec.
When it comes to sell in 5 years time, you will find way more buyers and better offers for the 20D.
If you go browse Autotrader you will see examples.
Maintenance wise, the n54 has its common issues to be looked at, that you don't have with the 20D.
The gents on here saying the 20D isn't that economical in very short and stop and go stops, if you think between 9 and 10L/100km is bad, what do you think the 35i will be like in the same condition's lol.
Yes the 35i is amazing performance wise and that sound is to die for, just watch the vids of Anees 35i and you will be seduced. If you dont worry about consumption and have an unlimited fuel budget than the 35i will be a blast.
A stock 20D is a tad under powered, but after a remap, it totally transforms the vehicle. I thought the acceleration up to 100km/h would mostly improve but to my surprise the highway performance improved even more. acceleration from over 120km/h was way better than i expected. Its just an effortless drive having maximum torque rite from pull away revs. My consumption has never gone over 9.6l/100km and the best i have gotten is 6.2l/100km long distance and not even trying an econo drive.
My Advice would be for your budget by the cleanest 20D you can find (especially if you can get Harman kardon sound (the base sound system is terrible) Or if you patient get a 30D. It honestly is a perfect car. Worth a premium. It is a buttery smooth engine, boat loads of torque, sounds great, and just as efficient long distance as the 20D.
Regardless of the one you chose, keep us in the loop and create a shoowroom thread of your choice, so we can see the nice new beast. Good luck with your search.