Both of those options could certainly work. Both are dehydrated powders that consist primarily of dehydrated cheese. Which one is better is going to be a matter of taste. The Kernel Seasons is specifically designed to go on popcorn, so it's more promising. However, it also adds a bunch of extra spices, so if you just wanted "cheddar" flavor, it's not your best friend.
The Mac and Cheese packet is also primarily dried cheese. However, it includes starch and emulsifiers, which might give the popcorn an unpleasantly gummy mouthfeel. You'd need to try it to see.
Other things I would personally try if I was substituting here include:
- Cheddar low-carb cheese crisps, powdered in a blender
- A mixture of dry "grated parmesan" and brewer's yeast (if you live near a natural foods store)
Good luck, and have fun with it.