Hi Tan,
It's a great position to be in!! My other half runs a 58 plate 2ltr Z4 and I have an '06 987.
I am sure someone will come along and talk about the technical differences between the two cars and how tthe location of the engine affects handling etc etc but for me it is a very simple thing. I get in the Boxster start the engine and even on the worst of days it makes me smile. The boxster sounds like a proper sports car , looks like proper sports car, drives like a proper sports car.
The BMW is a good car but for me it is very uninvolving, the exhaust note is subdued, the handling okay but the run flats aren't great, roof not as well insulated from noise - it's competent at what it does but just lacks the polish and theater of the Boxster. Think shrunk 3series with soft top - not out and out sports car
In the snow at the start of the year the BMW was unuseable, everyone I know who has a recent BMW on run flats has problems with it in snow - I owned two 3series' before they introduced run flats and they worked perfectly happily without resorting to snow chains - so i am guessing it has something to do with the tyres - oddly the ones fitted to the car are described by BMW as summer tyres and shouldn't be used in temperatures below 5C not exactly great in the UK where seperate winter tyres definitely aren't the norm as they are on the continent/ nordic countries. Had no problems with the Boxster at all, although getting the doors open sometimes proved iffy
The one big negative on the Boxster is that if you don't use it everyday you will need to plug it into the mains - last time I looked it wasn't an electric car!! The BMW will survive for weeks without even a sniff of the battery charger
Where the BMW really scores is running costs, the Z4 averages 30+mpg when used on a 20mile round trip commute in the rush hour, the Boxster averages 23mpg (although I am not that gentle with it). Servicing costs on the Z4 are also much better although the gap narrows if you are doing the work yourself. Also insurance is significantly cheaper
Before buying the 987 I did look at a 986, the big differences which made me choose a 987 were servicing interval (twice as long) , interior styling much more modern, factory fit satnav much better, glass rear window (although a 986 after 2003 will have one of these).
At the end of the day it's personal choice - the other half hates driving the Boxster for the exact reason I love it - the exhaust noise scares her!!!
Cheers
Paul