I guess when you say 5k cash, you mean they are going to give you $5k on top of what you owe. In that case, I say it really depends on what the total would be, and to make sure you're getting a good deal out of it.
As far as buying vs leasing. It sounds to me that your wife is telling you that when the baby comes (congratulations btw) that you'll have to get a four door car so youre just preparing for the future and getting one now. With that in mind, why does that automatically mean lease? Whats wrong with financing this car, and keeping it for the long run considering this is what your wife wants you to have when the baby comes. Now, what u want out of the car comes into play.
1. you wanta new car always
2. Dont drive too much
3. Dont care about ownership
4. Lower monthly payment
1. You prefer to get rid of the payment evnetually and have equity/own your car
2. Drive alot
3. You want to mod your car
As far as whether you should do it now or not, that depends on you. yea youll get more money for it now, but then youll have a sedan until the baby comes. would you prefer the comfort of not having to worry about the baby, or would you rather enjoy your car while you have it and then jump in a sedan when the time comes.
me personally? if the deal is right id prolly jump in the new car, but thats prolly cuz i like getting new toys