Sillage is how far your scent projects, and how detectable it is to people around you.
Longevity is how long it lasts on your skin until the scent fades away.
They often go together, but not always. It's possible to have a scent with, for example, very little sillage (so only you can smell it, and only by sniffing yourself) and enormous longevity, so that you can still smell it the next morning. Or a citrus scent that goes on HUGE so that all your neighbors know when you apply it, but is gone by lunchtime -- great sillage, terrible longevity.
The one true rule is that anything you HATE will have massive sillage and last forever, no matter how much you try to wash it off!