Just a bunch of random (and perhaps odd) thoughts from my brain...
1. I'd always say to keep the EX since it's paid off.
2. I typically don't spring for extended warranties, especially on cars with reputations for being reliable. From the car's in which I've been involved with the purchase of an extended warranty, the repairs within the extended warranty period never came close to the warranty prices. Yes, there's always a chance that an expensive problem may rear its ugly head, but I'm willing to play the statistics game and not buy an extended warranty.
That being said, whether you get an extended warranty or a new vehicle with a warranty, it does bring peace of mind. It's up to you to determine how much that peace of mind is worth. Also, if both you and your wife's jobs absolutely require reliable transportation (or a loaner vehicle), this may have some sway in your decision.
I'd be leery about purchasing an extended warranty that covers a vehicle over 8 years old and 100K. I'd guess that the options out there are not really worth the cost and that it could be difficult to work with when submitting a claim; and who knows how long the underwriting company would be around for.
3. While you could ask us all the questions you want, the decision will be between you and the wife. At this point in my life, I see automobiles almost as appliances that need to reliably do their job and meet the daily needs of family and household duties at a reasonable cost. Better performance and extra features are just icing on the cake for me. My wife likes functionality and reliability in her cars as well, but she's a lot more easily swayed by the styling and amenities of a vehicle. She's willing to pony up a bit more money for something nice. While it's a challenge, we usually end up finding a happy medium. In your case, your son may be the tiebreaker vote, so you might end up keeping the EX.
4. No matter which option you choose, you just might lose out in the end. Your wife might decide that she likes the nicer vehicle and end up taking it over as the car she drives most of the time

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