I had similar experience.. I drive 4.8 right now, and decided to test-drive 35i... Even though specs say it has same 0-60 of 6.4s, it was NOT. It felt slower, I had to push engine much harder to make it move... I also happen to have 2010 335xi, with almost the same engine as new X5 35i -- the 3-series is crazy fast, but comes with half weight of X5 though. For me 35i was a no-go...

So I had to make a decision of basically going to 5.0i or 35d.. In the end, given lesser base price and $4500 credit, the 35d came out to be almost $10K cheaper than 5.0i ! Not to mention fuel economy, which is big deal for me since I drive to work almost every day... So, 2011 35d should be in my driveway in a week +/- 2 days.

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His : 2013 X5 3.5d Deep Sea Blue/Black/Aluminum with ALL packages, 20" 214"s, running boards, Adaptive Drive, Spare Tire, BMW Apps
On Order : 2016 X3 3.5 MSport Silver/Mocha with DHP, CW, Prem, Tech, DA, Lighting

Hers : 2015 328xi MSport Silver/Red

Prior:
2008 X5 4.8
2011 X5 3.5d
2010 335 xDrive
2013 335 xDrive