1 APS (Attack speed is irrelevant for this relative comparison)
Fight duration 10s 15s 20s 30s
Vigilant watcher 175% damage 2450 4725 7350 12600
Spitfire 175+30% damage 2460 4510 7380 13530
Example say for 20s
Vigilant watcher will have one sentry do 175%dmg for 20 seconds. A 2nd sentry do 175% dmg for 14 seconds. A 3rd sentry do 175 dmg for 8 seconds = 175x20+175x14+175x8 = 7350
Spitfire will have one sentry do 205% dmg for 20 seconds. A 2nd sentry do 205 damage for 12 seconds. A 3rd sentry do 205% dmg for 4 seconds = 7380
1) Spitfire rune fires rockets at the same rate as your attack speed. Can anyone please confirm this. I find this one very hard to test.
2) No ballistics rune.
3) Sentry is placed immediately after cooldown is finished.
Conclusion: Both are very similar with the difference being less than 5% either way. For very long fights Spitfire wins. Overall, I would prefer Spitfire on the basis of this analysis.