If you're going to keep the 35i, I'd hitch that. The X5 is rated to tow the same regardless of the engine. I still think the diesel will tow more "effortlessly" and efficiently than the 35i, but ultimately they will be equally as capable.

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2012 X5 35D - EGR delete and Stage 2 tune
2003 M5 - Imola/Caramel, Dinan S2 suspension, Alpha-N tune
1990 M3 - CF Airbox, Alpha-N, Schrick 284/276 cams, VAC S2 head, full GC suspension
1991 M5 - Black/Black - stock
1991 325iX - Winter beater

Retired: 1999 328is 5MT, 1994 525i 5MT, 2003 530i 5MT, 2012 X3, 2006 325i 6MT, 2001 M5