I have a 2012 Mazda3 that I recently discovered has a leaky tire. The pressure light came on yesterday, and we located the low pressure and added 10-15psi. This morning I drove to work, but the pressure light didn't turn off. I checked the pressure when I arrived and the bad tire had dropped back down again. The leak isn't bad enough to completely flatten the tire, but it also isn't slow enough that a refill will last me a week.
I haven't had a chance to try and locate the leak itself yet, but when I do, I have a couple options for handling. I have both a fix-a-flat and a spare donut in my trunk, as well as a portable air compressor. It seems that using the fix-a-flat would be much easier than changing the tire out. Reading this answer on another question, it sounds like fix-a-flat also would last quite a while, long enough for me to get the car to my regular mechanic on the weekend.
My spare is the one that came with the car 6.5 years ago, so it's also reaching the age where it should maybe be replaced. However, I understand that it may be far safer to just change out the tire, even if it is a pain on a 90-degree day.
So, to bring it all to a question:
If one has the option for both fix-a-flat and a spare tire, in which situations would each be recommended?