I made the mistake of driving a bmw135i, coupe, 2009, 6000miles, tight as a drum and loved it.
Downsides= poverty spec, petrol costs (I think slightly worse than I get at present), and an insane ability to lose your licence.
to pay circa £120 more per month to finance (not too much of an issue really). Runflats.
Upsides= newer car, a change, Fast as fook, LSD.
My gut feel says hang onto the gti she is running like a dream and in 2 years, after saving for a maintenance pot, get myself a good 911 c4s.
I'd still like to try a 123d and see how I'd get on in the real world with one though I do have a hankering for a newer ride, but after coming from a fully loaded gti with every toy it's a hard step to go back to a buggy (if you could ever call a BMW 135i a basic buggy).
The grass is always greener etc.