I was in a similar situation last month... either 4.8i or 3.5d... I did trade in a 335, so I know what that turbo, Normal aspirated engine can do,
(given the weight, car,vs truck..etc)...

I was and thought 3.5d all the way, until I drove the 4.8..
PROs and CONs, IMO!!!

PRO Diesel:
Torque of the diesel.
Distance between fill ups
MPG.

CON:
Sound...
My choice was W/O SAV pkg, 20's, fender flares,
Price of Diesel,
Higher Sticker than N.A
$4,000 more than 4.8

PRO 4.8i
Balls to the wall V8!
great Passing gear, effortless speed
SAV Pkg, w/20's, etc...
That rumble of the exhaust

CON:
Distance between fill ups
MPG

The 4.8 was a CPO, with what I was looking at this was my choice... The 3.5d is a great engine, but I wanted that HARD look of the wide stance, granted I could get that added later, (for about $5,000 out of pocket) but I wanted the total pkg as one...

If the 3.5 did have the SAV trim and was cheaper... then I am not sure what I would of done...but that was not the case...

Not sure if this helps, this was my situation last month...

ak

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14 335i. Ferrari CX-60, 11msport 328, X5 4.8i, 08 335i Coupe, 10 C4S NIB, 06 X3 Sport, 08 550 CLS, 01 CLK 55, 04 Saab 93 forgotten, 06 Crossfire?, 96 M3, 94 968 cab, 99 C43 AMG, nice, 96 C280, 89 325is drifter, 93 190e 2.6, 95 Jetta VR6, 91 Corrado G60, 90 BMW 740il, 88 Accord Coupe Lxi, 87 Mazda 323 GT and first was a 84 Mercury Lynx Rs.