This is pretty much just a personal preference thing. I personally leave my system on but make sure the display is set to blank when not being used. I do that because I often want to access files on my system from other machines I own.
From a cost standpoint, it will cost more to leave it on all the time then to shut it down when you’re done for the day.
As far as the computer longevity, I don’t think it will make a big difference either way.