I rarely see Spies only go for the Dispenser first, and then while I'm repairing that, go for my Sentry, and then kill me...that's pretty much never happened to me.
My dispenser and sentry are almost always right next to each other...so when I get sapped by a Spy, it's pretty much simultaneous. I'm constantly hitting my sentry with a wrench because it's getting shot at...so when a Spy comes and saps my stuff, I just keep repairing my sentry.
I find that I'd rather lose my dispenser for the sake of my sentry. I fix my sentry, and I'm constantly moving, so I very, very rarely get backstabbed, and once my sentry is fixed, I shotgun the Spy. Then I'm usually good to rebuild the dispenser. If the Spy tries to stab me, he'll just get shot by my sentry.
That way, I stay alive, and my sentry is still up. Staying alive as an Engineer is more important than keeping the dispenser up, of course, because I can always rebuild.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )
