I bought my 2005 3.0i two years ago with about 42k miles on it. At the time I could go either v8 or I6, no preference really (actually more inclined to go V8 route due to better power). I called around and asked a few local BMW independent repair shops about their view on reliability and maintenance of V8 vs. I6, and asked if they accepted any third party warranties, etc.

Basically every one of them (all legitimate experienced businesses advertising their shops on local Craigslist and such) told me that with an 3.0, I would not need to worry about a warranty, but that I shouldn't get a V8 without such warranty. All of them, (I called about 4 different shops), said that in their experience, V8 was more of a risk than 3.0.

I went with the 3.0 and have been pretty happy with the decision so far. (No warranty, good reliable performance.)

I did test drive a 2007 4.8i (new style V8) a few months ago as I was considering an upgrade to the new body style. I then test drove the new body 3.0i as well. Frankly, after test driving them both back to back, with warranty, I would still get the 3.0i. The acceleration was better in 4.8i from stop, but at freeway speeds, passing acceleration was very similar in my opinion. Now, obviously, 4.8 has more power than 3.0, so I am not denying that. What I am saying is that 3.0 has to work a bit harder to achieve the same acceleration, but it seemed VERY WILLING to do just that. Take into account the number of times I will realistically have a need to push it that hard, and even the cost difference between 3.0 and 4.8 could not be justified (for me and my driving purposes.)

I have had multiple trips to LA from San Jose area, with full trunk and 4 passengers in the car (including me), and even going up the mountains close to LA I saw no problem with accelerating/passing uphill. Again, the engine is working harder if you choose to push it, but given the number of cops on the roads/etc., under generally normal driving conditions, the 3.0 was not lacking by any means. Although if I were towing any heavy boats, I think wouldn't have been happy.

If I were to buy an X5 today and keep it only for 4 years under initial free maint./warranty period, I would go for a V8. Who doesn't like to have more power? However, even if I wanted to go the new car route, I would need to keep such vehicle for at least 10 years to justify its cost, and that alone would make me want to get the smaller 3.0 non-turbo engine (but that option is no longer offered anyway, so diesel would be my preference over V8.)

I really think you will be happy either way. V8 *may* give you more headaches, but will also give you better performance. Test drive them back to back, a few times, and decide for yourself and your driving needs which one fits your needs better.

__________________
Currently driving: 2011 E70 x35i , 2011 E90 335d
Previous BMWs: 2007 E90 328i , 2005 E53 3.0i, 2011 F10 535i (6spd manual)