Good write-up TMP, interesting to read.
I came from a 135i (E82), having had a 335d (E92) before that.
The 135i was a fun car, for sure, loved the sound of the engine (when you were in the mood), little pocket-rocket. The sound - both starting it up in the morning - and when motoring on, did get a bit tiring for me though, as it wasn't something I wanted on every single drive. To overtake a car in a tunnel, I didn't want to scare them half to death, just to subtly overtake, but all the drama and soundtrack eventually became grating for daily driving; and I personally felt the engine - needing to be rev'd more than the big diesels I was used to when pottering about - sort of "egged you on" and I personally found it difficult to drive as smoothly or as calmly as the 335d, and found myself driving more aggressively than I'd like most of the time. I also felt I'd stepped back in time a bit to my MINI Cooper S Works/Golf GTI days when I was younger, the car was the next logical step up from those I guess.
In contrast, the F30 330d is very similar to the E92 335d: effortless, quiet, just gets the job done without shouting about it. The torque available is great. I agree that it's difficult to get going (the 335d was the same), and when on WOT, the diesel obviously runs out, while the 135i keeps on building and building; but for the majority of normal driving, I find the 330d a much better drive, with more accessible power, unless you're wanting to drive like an idiot (which I found happened more than I'd like in the 135i!).
Economy-wise, I'm getting 34mpg in the 330d vs 22mpg in the 135i in the same town/traffic driving. On the motorway: close to 50mpg in the 330d and I got around 35mpg in the 135i, so a good improvement. I also like the different driving modes and have found I use them a lot more than I'd thought: I stay in Comfort most of the time, switching to Eco-Pro when in either heavy traffic or when cruising on the motorway; and occasionally switch to Sport mode if I get an open stretch of nice road, and/or to whip past a couple of cars should the need arise
Interior quality is much better obviously, and the car is more refined generally (though wind and tyre noise are still higher than I was expecting!). I'm also loving the new tech e.g. HK/new Nav/Adaptive Lights/HUD.
The suspension (I have Adaptive M Suspension as well) gives a much better ride in Comfort mode; and it firms up nicely in sport mode. I'm also happy with the steering (non-VSS); my 135i had hydraulic steering, which I thought I'd miss, but I find I actually like the lighter steering in Comfort mode (which is still very accurate and precise, despite being light); and it's nice to have the option to firm it up in Sport mode. There is definitely more body-roll than the smaller 135i, but on the other hand - due to the longer wheelbase at least in part - it feels more stable, planted, and controlled, particularly at speed, and I personally prefer that.
The 330d is - for me - the better car if this is going to be your only car. It's much better for longer journeys, more practical with the extra space and 4 doors (for me, compared with the E82 coupe), and gives you a more appropriate daily drive, with the option of making it sportier if you're in the mood. I loved the 135i and will miss it - my girlfriend particularly so, as it suited her better and I'm having a had time selling the 330d to her [it's our only car]! - but as we only have the only car, for our needs, the 330d suits us much more. All are exceptional cars though - M135i even more so than the 135i obviously - just for different needs