Quite welcome. I will say that from my experience, there is an immediate and obvious difference when you step on the gas, but other than the 50i kicking you in the butt - you will notice this immediately - and the 35i more like pulling you along quickly, there is no difference between the two. In fact, the 50i literally wants to jump off the line when you touch the gas, and to me this was rather ANNOYING, despite the fact that all I could do was laugh and laugh while it rocketed from one light to the next.

Best I could tell, handling is more or less identical, but I have to add that the 50i I test-drove had the M package (the 35i did not).

I thought the 35i was perfectly adequate - I am not an overly aggressive driver and I don't need any tickets thank you - so the 50i was overkill. It's very nice to have that sense of sheer power, no doubt about it. The more I drive the 35i, the more I notice that the X5 as a whole seems optimized to perform better the faster it goes. Both models will go from say 70 to 100 before you even realize it (had I not had the speed alarm set I wouldn't have) and you won't even feel it. It's astounding.

I would recommend the same thing I did: drive the 50i FIRST, then see if you miss anything in the 35i.

Some folks really live for the kick in the pants of such a powerful engine, but you have to decide whether that's worth like 5MPG (and reduced range - gas tank is the same size between the two) and another ~$8K to you or not.

I know the BMW purists hate the thought, but I spent the extra $8K in creature comforts (and extended warranty to 100K miles).