I have fought with this decision and in the end it usually costs as much to drive the old one as the new one. If you buyout is 42k and it will only have 1 year of warranty left on it, you will need to have it CPO'd, which could cost a couple grand, but even if you work it in and pay 42k for it CPO, you would be looking at 5 years of payments at almost $800 a month (and 5 years is too long to be paying for the car - you should have it paid for by the time the warranty expires which is 4 years and would cost almost $1000 a month).

So let's assume you paid it off in 4 years, now you have a 6 year old car worth 10-15 grand (just a guess) that has no warranty left and will be expensive to fix and at the age and miles that will require repair.

You could lease a new 35i pretty loaded up for $700 or $300 less a month than the payment on the old car (and you get a new car), it is just as fast as the old V8 (although the V8 sounded better for sure), you get all the new updated stuff and the facelift. You also save $14400 over the payment you will be making (although you will only have the car 3 years instead of 4).

All the math is lot's of guesswork, but ultimately it will cost you basically the same to drive the new one as keep the old one and in my experience there are only 2 senarios that make the owning worth considering - first is the mileage, if you drive too much, the lease is too expensive or just not possible (doesn't look to be an issue with your driving, if you only did 22k in 3 years, then you could do a 10k lease and save some money) or the second scenario is that you keep the car for 10 years and hopefully get lucky and don't have any major repairs.

If you like to drive new and are not going to keep your old one for 10+ years, then just getting another lease is probably no more expensive for you and let's you drive a new fully warranted vehicle.