The fun to drive factor is there in the x5 4.8i, but its not a 335i.
If what you need is comfort for longer drives then the x5 will be great, but you for sure want the sport package and the comfort seats. We took our new x on a 700 mile one way trip and its was effortless with no discomfort. wel, other then the kids waking up.
If you are chosing between the cars then I would go with the x5, because of your complaint. you will still get plenty of fun factor out of it. you may miss the 335 but not much.
I got rid of my 335 two months ago and got a jetta to drive untill my M3 gets here, and its teribble, but whenever the wifey is home I take out the x and its great.
hope that made sense and helped you out

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M3