Seeing as you are at the coast, 125i is quicker off the line and to the 1km sprint than the 120d, stock vehicles compared, BMW factory spec sheets say so, with a higher top speed. Let the facts show that. Yes I understand in real world driving the 120d will pick up speed quicker in gear acceleration due to the torque. If you looking at the manual, you looking at 0-100kmh in the 6sec region vs 7sec on the 120d.
I will present the other side of the coin, seeing as this straight six gem, is now not the most popular chappie in the room these days, but still has alot of appeal. You probably think by reading the opinions so far that the 125i is gutless and powered by a 1.3L engine.
As far as the engines go, I can tell you the 125i is a 3.0l, and is different to the one in the E9x 325i and it shows in the way the cars drive. Look it is not the most powerful 3.0L engine, but the reasons why it was used are mentioned previously due to power to weight ratios. Also the engines have different power delivery, probably due to different engine mapping, especially in sport mode. I can compare as my previous was an E92 325i, which was pretty sedate.
If you spending that much money, and there is no difference in price and performance is stronger in stock guise with fuel economy being the only difference,
you have to make a decision here:-
1. Do you really want to watch fuel economy in a sporty vehicle such as an E82 coupe?
2. Can you live without the traditional BMW refinement and the exhaust tone of a straight six?
3. Can you live with a clattery sounding diesel engine in a 2door sport coupe?
I choose the 125i as I know the way I drive would not suit the engine in the 120d, no matter how much torque it has or how good economy it returns. I want to rev the engine up, hear the exhaust tone at 6000rpm, the diesel only revs up to 4000rpm where it delivers its peak power, and that's not fun for me. I will always be wanting more revs. Also having instant lag free torque available and the ability to punch the vehicle out of corners with revs is something I enjoy as well. I've owned a 320d E90, so I understand the power delivery of the engine and its limitations. The diesel engine application is more suited to familly sedan in my opinion, providing fast, effortless and quiet pace when on the move. I also have no intentions of enhancing performance as I know the the gains may not be worth the money to invest. Look the 120d probably has more potential as a tuneable car, but to risk modifying a diesel and then blow a turbo in the long run is a risk I did not even want to even think about as I intend on keeping my car well past the M/plan expiry date.
Don't be scared by the horror stories about fuel consumption also, I'm averaging 11.4L per 100km in heavy traffic driving, it probably will drop if
I travel more on the freeway for a longer distance. To achieve consumption of 14L, you probably driving like the Stig everytime you get into the car.
I would think most of the opinions posted here are from people living up in higher altitude Gauteng and where straight roads dominate and straight line performance is the be all & end all of performance. I'm formerly from Durbs where the roads are not straight and where you can really enjoy a cars handling feel through the twisty roads. The feel I get when taking this car by the scruff of its neck through a set of corners is just simply remarkable and something a diesel vehicle can not match. It is not as perfect as an E92, but it keeps you thrilled, that little bit of a slide which you can correct etc. You have to think about wheather the diesel engine will suit the sporty nature of the car.
I enjoy my car for the way it corners, the feedback I get through the steering wheel, power delivery at the bottom. Ultimately I would have wanted a manual but for practicality of sharing a vehicle with my wife opted for the auto. Look if I had choosen a 120d, I'd probably save half on my fuel bill, but I dont care about that, I enjoy the drive the car delivers, sportiness, compactness, power which I feel is adequate for daily driving in traffic etc. It is one of those grin inducing drives, when driven to its full potential!
If you want more performance I suggest the 135i, its a different animal all together, but this is about the 120d and the 125i.
Sorry about the long post, my car makes me do these things..........lol