ok, heres the thing...
If you can afford to get a bmw and be comfortable with the payments, please get one. You wont regret it. Much nicer to drive than a polo.
However if you think that having another car to pay for monthly is not in your best interests, dont let anyone try to persuade you to buy another car.
Sometimes you have to gear backwards so that you can move forward better.
you could keep the polo, and have a freehold car or you could get the BMW. Really up to what your wallet can be happy with.
Dont worry too much bout the wife. take her for one test drive in a BMW and shell be begging you to get one. Just make sure youre all cool with the decision.
The thing about this forum is that were not here to tell you what to do. We offer advice, sometimes slanted towards BMW's (This is a BMW Forum of course), but mostly level headed advice. Hope this helps.