Daaaaan
I think you may be getting too cought up in whether you have the M54 or N52 engines, whether you have auto or manual, and what the actual fuel consumption is etc, etc, etc
A big element of any Z4 is the fun you have driving them and the smile on your face when pulling up to a set of traffic lights with the roof down (sorry Coupe owners

)
I have to admit you made a good point here...
Your choice is a simple one. If you prefer the look of the facelift, get the best one of those you can afford, if you prefer the pre-facelift, get the best one of those you can afford.
Simple economics says that you will be able to get more options and a bigger engine on the older model. It is just what your preferences are.
The dilemma I had: "to choose, or not to choose the facelift", was a matter of engine upgrades that were made (M54 to N52 2.5i with double VANOS, and the launch of the N46 2.0i engine) and of course reliability issues that got fixed. It was not at all about looks, but i understand your point of view...
Either way, you will love owning a Z4 whatever model you choose, don't worry about it, just enjoy test driving a few different cars and choose the one that "feels" right for you.
Don't forget to post pictures of it when you get it.
I attached some pictures of the 2 cars i have to choose from (black one: 2.0i 150hp 2007 45000 km, white one: 2.5i 177hp 2006 39000km...the rest of the specifications are practically the same, the white one has parking sensors, and the black one has an alarm system...but that's unimportant, they can be both fitted later)...price is the same
... what do you say? should i be "glamorous"
and get the white one, or should i be "knight rider" :evil: and get the black one? does white look like " football players' car colour" on a roadster?