eeeThe difference is the diamond back isn't a new strategy. It's literally getting a reward for what you were going to do anyway. If scout got a weapon that made him insta kill after getting a meatshot, it wouldn't change scout strategy, it'd just be a scout doing his job and getting a buff for doing what he was already going to do. There's better ways to buff spy, giving him a babymode ambassador isn't one.
l'etranger
#1 at getting stabs (more cloak)
#3 at killing things with a stab (can't 3shot meds)
#3 at killing things without a stab (can't 3shot meds)
The l'etranger is still the best at getting stab kills. Stab kills are the most important part of 6s Spy, a gun is a backup plan. Having a shitty gun doesn't matter that much if you just hit your stabs.
diamondback
#2 at getting stabs (no extra cloak)
#1 at killing things after a stab (1 crit + 1 regular 2shots meds and scouts)
#3 at killing things without a stab (can't 3shot meds)
The Diamondback doesn't have extra cloak, so you should probably only use it on small maps/points or maps with lots of ammo, where you don't need the extra cloak to get behind. It's best for players confident in their stabbing because it's easier to turn single kills into doubles, but if you don't land a stab, you can't 3shot the medic or 2shot after a butterknife, and that really sucks.
revolver
#2 at getting stabs (no extra cloak)
#2 at killing things after a stab (3shots meds at cqc comfortably, knife + 2 bodyshots kills too)
#1 at killing things without a stab (3shots meds at cqc comfortably, knife + 2 bodyshots kills too)
The stock revolver is basically a different style of Diamondback. If you miss a stab, you can still 3shot the medic easily factoring in falloff and regen, and 2shot him after a butterknife. You lose the ability to stab the pocket or whatever and 2shot the Medic though.
ambassador
#2 at getting stabs (no extra cloak)
#2 at killing things after a stab (1 crit + 1 regular 2shots meds and scouts, no setup required)
#2 at killing things without a stab (1 crit + 1 regular 2shots meds and scouts, no setup required)
The Ambassador can 2shot Medics at point blank without a stab. It can also 2shot Scouts at close range without needing a stab and kill Demos and Soldiers in two headshots from long range. However, it's also really hard. 6s Spy is about getting stabs first and foremost, and then dying ASAP so you can switch to your main. So the range of the Ambassador usually doesn't apply.
basically l'etranger is for stabbing, diamondback is for 2ks with people good at stabbing, revolver is for guaranteed med kills for people bad at stabbing, and ambassador is for people good with the ambassador
i wrote this basically to say that the diamondback is on par with the other guns except the l'etranger, which is still leagues ahead