Kudos to you and your wife, 1canvas, for putting more thought into the purchase beforehand than most do.
1. Buy quality. You're already ahead of me on that; the Fox Labs brand you've settled on is top-notch.
2. You and your wife are in a better position to pick a delivery system than any outside consultant, because you are the ones in the best position to assess your "threat profile." Your greatest exposure to threat is after dark in an often windswept parking lot? Streamer will give you much more range and accuracy, and is much less likely to be blown back into your own face. (When under attack, perhaps rapidly circling your opponent, no human being can be expected to keep track of wind direction.) More likely that the attack won't be perceived 'til it's up close and personal? A fogger gives you the best, fastest chance of getting the OC into eyes, nose, and mouth of the attacker immediately. You might even want to consider a "wardrobe" of different sprays to carry in different situations and seasons, much the way so many of us change guns depending on weather and "dress code" demands, or use a different pistol for a hike in the mountains than for a walk in the park.
3. Memorize the manufacturer's instructions and particularly the expiration date. Keep your spray fresh the way you'd "keep your powder dry."
4. Get training! Movement patterns off-line of attack and things of that nature are critical. Pain can cause rage, and even if the OC has forced his eyes shut, you don't want to be where an enraged man last saw you if he charges.
5. I would strongly advise a class in which you get sprayed yourselves. First, you need to know what it will and won't do. Second, you need to know that you can fight through it yourself; people employing pepper spray often get unintentionally dosed. Third, if you're accused of using excessive force, it's a powerful defense to be able to show that you didn't put your attacker through anything you didn't voluntarily put yourself through first.
Good luck to you both. Stay safe!