Well, I've had the E90 330i since it was new (wife's main car) 10.5 years ago, so I've had a long experience with E90 rear seat accommodations. The E90 M3, which I've had for over 3 years now, is identical in room in the back to the 330i with just softer, more comfortable leather I would say. In any event, it is noticeably less roomy than the E39 in back seat in two areas: headroom and width. I'm 6'2" and somewhat tall in the torso, and sitting the back of an E90 I'm not very comfortable for a long trip; however, short trips/usage is no problem, and it might be fine for shorter people, kids, etc.
Front seat in the E90 is much more comfortable to me than the M5. The sport seats fit me perfectly in the E90, have adjustable side bolsters also, and the driving position is more comfortable for me, especially the relation of the seat bottom height to the floor/pedals. All of this is subjective and highly dependent on one's size and proportions of course.
Re monoballs -- most people with stock upper strut mounts in front report little to no increased harshness. However, in my case I already had the Ground Control camber plates/mounts, and when I added the monoballs, the NVH definitely went up over sharp transitions such as Botts markers, broken pavement you can't drive around, stuff like that. It's not "bad" per se, just noticeably louder "bang" over those than without the monoballs. The car felt wonderful to drive however after the monoballs which was the last mod I did to the suspension 8-9 years ago now; sharper turn-in response and feel, and during heavy trailbraking with R-comps mounted, the accuracy and feedback through the steering wheel increased significantly.
Regards,
Chuck