In reply to the original question of E61 or E91? ... The bottom line: Neither model are that different in either size or performance (that is with any engine over 3L) except in one area - The far superior ride quality of the 5 series Touring on Sport suspension.
To elaborate:
I own both the E61 and E91, both 3L diesel and ask myself this question a lot (!) Which should I sell? Btw, I also own an E46 Touring (NZ new 328i SE with MSport trim and seats and the perfect standard suspension) and have had it for 12 years and neither car is a great improvement over it.
Size: I've been over each of them with tape measures and there are very surprising results. Despite what several people have said above about the 5er being huge cars, weirdly they are in fact almost identical in every external dimension except length. Yes, it's true. Even the interior space.
Exterior: E61 is only 17mm wider and 317mm longer. Yep only 17mm mirror to mirror. The length appears as 100mm extra wheelbase and 100mm front and rear overhang.
Interior: The front elbow room (door armrest to door armrest) is 5mm more in the E61, B pillar to B pillar is 20mm more. Back seat width is only 30mm more in the 5 series but the backseat to front-seat distance is exactly the same with my driving position in each. Yes exactly same. THE big difference is in the boot space. The E61 has 200mm more length and because of the rear air-springs sitting under-floor rather than creating wide strut towers, the width is 100mm greater at max and min widths but also the widest part is squarer.
Engines and gearboxes: I have essentially same engine in each (160KW vs 170KW, both with 500Nm) and same gearboxes. The E61 was UK market so came with dpf in exhaust. This was deleted, so while the 3 series was marginally better performance when new (lighter), now the 2 are pretty much identical in performance. My E91 is NZ New and like all the NZ new diesels it was built to Euro3 spec so has no dpf (bonus!).
The 5 series really was built as a higher grade car. Trim, solidity, interior details, door heft etc. but the main area that all this shows itself is the suspension and chassis. In my experience, which is mirrored in other reports, the 3 series is far better on standard suspension (both the E46 and E9x). On Sport suspension the ride becomes way too harsh and fidgety on NZ roads. Motorways are fab, but anything else? No way. However the 5 series Touring is the opposite. The standard suspension is great but the Sport suspension has all the tightness, response and control while at the same time being the best overall ride of any car I've owned (there's been a few!). I have 19" wheels with 255/275 tyres and even catseyes on the m'way hardly cause a murmur. I can't personally speak for the E60 sedan because it has a standard rear spring, but I do know the E61's Sport-spec air-springs are superb. The E61 can lack feel through the wheel but the steering is well weighted and precise.
As a commuter, the E61 wins all the way. It's just far more relaxing when you want it to be. Stuck or cruising in traffic it just feels nicer. As a nip and duck city car though the E91 is super nice. It feels way more 'darty' and zippy. Of the engine options I'd def recommend searching out a diesel - the most reliable of the engines. The grunt and ease is astonishing in itself but the fuel economy is crazy. I get 6L/100k easily on the open road, and overall combined with urban and city hills etc it averages 7-8L/100. Crazy for a car whose midrange punch-in-the-back feels like a sports bike when overtaking. I also ride a TL1000R bike and while it def can't really be compared in outright power terms both the 530D & 330D are the closest I've felt on the open road to the LOW REV ease of overtaking that big bikes have.
