Fury cutter is strictly superior. The only time you'll ever actually use a Pinsir/Scyther in a battle is against an Exeggutor because of the double weakness to bug. Not only does Pinsir not get STAB with rock smash, fighting is weak against psychic types, so you're effectively losing 50% damage. Aside from Exeggutor, bug types are completely worthless in Gen 1 because every other grass type is also a poison type, cancelling out the bug weakness.
Few more anecdotes about gym battles.
On defense, you want the slowest charge move your defender has the hps to support. I went after a Blastoise with a Raichu this morning (Steamrolling everything with my A-team gets boring) and I was absolutely handing him his ass, and then he KOd me with hydro pump. If he'd had ice beam or flash cannon I would have won before Raichu got knocked out. I have a very powerful electrode with hyper beam, but with his low hps he isn't going to survive long enough to launch hyper beam, he'd have been better with thunderbolt. My hyper beam wigglytuffs are much better on defense than my play rough wigglytuff. A vaporeon with water pulse is basically nothing more than a meat shield.
On defense, avoid putting pokemon with a shared weakness next to each other. Lets say you have back to back Vaporeons and I bust out an Exeggutor. Due to Vaporeon's giant hp pool, my Exeggutor probably isn't going to beat both of them, but if he stays conscious on the second one long enough to fire off a second solar beam, I've gotten an absolutely ridiculous amount of value out of him. Staggering weaknesses, or even better intelligently placing counters to counters (meaning lead with a water pokemon, follow up with a fire pokemon, then a grass pokemon) is a generally better tactic. That doesn't mean use an 1800cp Flareon to back up a 2300cp Vaporeon instead of tossing a second 2300cp Vaporeon into the gym, or avoiding putting a Lapras next to a Snorlax because they're both weak to fighting. Intelligent judgement trumps general guidelines.
On offense, if you have the revives to spare, don't swap out a low hp pokemon. It's basically done for the battle anyway, so you might as well squeeze a little more damage out of it instead of wasting time fiddling with the menu while the defender continues to attack.
On offense, your pokemon can still launch tap attacks until the KO animation finishes, so don't stop tapping the screen. I got a mutual KO with my Raichu on that Blastoise from that earlier example even though Raichu went down first.
*edit*
Whoever was talking about Kingdra earlier, a lvl 40 perfect IV Kingdra will have 2671 max CP, putting it about on par with Gyarados. So not the best of the best but still a top tier Pokemon depending on what moves they give it, especially considering how much easier Horsea is to farm compared to other stuff. If you want to figure out what your favorite later gen Pokemon will look like when they arrive in Go, the formulas are here.
Pokémon Stats - Advanced | Pokemon Go