Performance = negligibly different, certainly in the 'more than enough' territory
Fuel consumption = LPG gives you an honest overall 30%+ reduction in fuel costs, so for me, the 6.2 would likely work out cheaper (although I would have to take account of the cost of moving my kit)
I accept that you have an unusual use case with respect to LPG, so I understand why you're minded to go for the NA engine. The improvement in fuel economy of the Biturbo engine over the NA engine in real world driving is probably around 15% - 20%, so that will help you work out the break-even point regarding moving the installation to a new (to you) car.
While the outright performance against a clock is similar, it is accessed completely differently. The forced induction engine pulls hard whatever engine speed, while the NA engine has to be "driven" (i.e. revved) to access the performance. I'm fortunate enough to have had a Biturbo E63 and my wife's R172 SLK55 with the NA version of the same 5.5 litre engine to be able to drive back-to-back, and in my view the forced induction engine suits the character of the E-Class much better than an engine that has to be driven at the top end of its rev range to achieve the same thrust. And before anyone says "the M152 isn't the same as the M156", I
know it isn't, but the power delivery is similar and by the time the NA engine has scurried down the gearbox and wound itself up, the forced induction powered car will be long down the road.
Various detail improvements: what are these? Not seen any mention of them?
Obvious changes are improvements to the functionality of the Driving Assistance systems and COMAND, while others are more subtle. For example the front door window lifts really struggled on many pre-2012 cars in cold weather. My 2011 E350CDI suffered this, and it was resolved by the fitting of thinner window glass and revised window channels while under warranty but there will be many cars that did not have the warranty work done. My point is that all models evolve during their production run, with detail improvements that are often hidden but add up to a better product overall.
My big concern is the complexity of the 5.5 over the 6.2, I'll be running this as an older car, likely as a long termer doing a fair amount of miles.
Just remember that while the engine may be the heart of the car, there's an awful lot more to consider than just the engine. My honest belief (based on ownership experience) is that the later cars are a better proposition - especially if you can run to an early facelift - but that doesn't mean an early NA car is automatically a bad choice.
) Forget the exhaust note, go on perfomance, and the engines speak for themselves.
(BTW, the video title is misleading. There are 5 defects)
