I considered lots before I changed, I did like the 5, but my annual mileage (thus my requirements) changed so I had been thinking about going back to a 3 for some time, here are some initial thoughts.
I think both are fantastic cars, but I like the E90 more, it feels more taught and planted on the road, the cabin is more compact that I feel more in touch with the car, which is what I like in a car. Both cars are good, the 5 was the Solid Edge, and had a more executive feel, while the 3 is pure sports saloon, with great ‘round town’ manners.
The ride is not as smooth with the 3, but I expected that getting the M sport version with stiffer suspension.
The 3 has the updated 3 liter diesel engine which has more power than the 5 had. Although the 5 had plenty of grunts, the 3 is a lot more urgent and sporty, I attribute this to the more powerful engine, sports suspension and a lighter car.
3 years ago I would not have dreamt of owning a diesel or an auto, but the engines in the 3/530d's mated to an excellent 6 speed auto have completely changed my mind. The torque is tremendous and feels never ending and I usually never notice the gear changes.
I also miss the space of the 5 on occasions, like family trips or transporting my golf kit. Boot space is not as good as the 5, I miss being able to throw all my golf kit and trolley in there without having to take a lot of care, or even having to put golf clubs on the rear seat, but I came live with that.
MPG is far better in the 3, (37.7) mpg compared to (32.4mpg) in the 5. On a run I can see 45mpg + in the 3, I never saw more than 37mpg 9 (Still Good though!) in the 5, and at £4.50+ ($7.50+) per gallon ity all counts :-) Because the 3 has a smaller tank the range seams about the same.
Although my 5 had more extras and toys, I don’t miss many of them. I like the Aux In for the stereo, I can play MP3 in all the disc drives and the professional Nav system in the 3 is a lot better than the business nav of the 5.