Having just moved from a 2008 4.8 to a 2011 50i, I'll weigh in, as I too strongly evaluated and drove the 35i. I've always driven and built high performance cars so my evaluation is a little more on the sporty side than most.
The turbo six in the 35i is a pretty remarkable engine. I'm fairly certain the 300/300 rating is conservative from BMW. It gives very good performance with impressive mpg's and will be more than adequate for all but enthusiasts. The engine does have less torque than the 4.8 and seems slightly less responsive at lower rpms as such from mild turbo lag. This is mostly noticeable at initial take off from idle. I noticed no turbo lag at highway speeds. DS mode will keep the turbo's spooled enough for instant throttle response.
One thing that surprised me was that the front end weight savings was noticeable with the six. Transient response is a little better in the 35i than the 4.8 and 50i, all things being equal (sport packages, wheel/tire sizes, adaptive drive, etc...).
This X5 is my wife's daily driver and she and I both would have been perfectly happy with the 35i, but we did opt for the 50i after driving it. The engine is simply amazing all the way around and is perfect for this type of application and gets a little better mpg's than the 4.8 did. In true BMW fashion, the 50i pulls very hard all the way to redline. The 1200 mile break-in period was VERY difficult, but fortunately my wife drives alot, so I only had wait a couple of weeks before stretching it's legs. We decided against the X5M since, for us, it's mainly the family car, but the engine is very addictive and now I'm looking forward to a Dinan tune they will bump the power to that close to the M.
BTW, in case anyone is interested, last night I recorded a 0-60 of 5.19 seconds, and an amazing 0-50 of 3.82 seconds in our 50i. That's faster than both a two year old 911 Carrera and a Cayman.
I don't think you can go wrong with either engine when moving from the 4.8, it's more a matter of personal preference and how much performance you desire.
