Really, I think the discussion goes beyond just those two. There are 4 units that can broadly slot into the same S8 shot role. My opinion of them:
Deffkoptas: The fastest/most manuverable of the choices, so the best at getting that nice side or rear armor shot. With the buzzsaw, also the best in close combat. Arguably the most fragile choice (depending mostly on how good you are at keeping the other options in cover). However, one of the most expensive choices, the most if you add in buzzsaws (or really even one buzzsaw)
Rokkit Buggies: Faster then kans and cheaper then either kans or koptas, these guys can do ok. Unlikely to get a rear armor shot, but side armor remains a solid possibility. As an AV10 open-topped squadron they are literally the definition of a fragile vehicle, but even the most fragile vehicle is still the equivalent of a T6 model with ~50% chance of not-dying even if wounded, even before a cover save. Unfortunately, packs 0 close combat ability, and what ork doesn't want to get up close and personal for some serious crumpin'
Rokkit Kans: Expensive - up there with the koptas. And not fast enough that a rear or even side armor shot is likely. On the plus side, AV11 moves them out of the range of all non-tau/necon small arms fire increasing their survivability, can run to otherwise help increase mobility, and they even at WS2 they pack a great close combat punch with the dredd close combat weapon.
Big Gunz Kannon team: Fragile, weedy, poor mobility, and dead they second something charges them. However, I consider them a contender and one of the more under-appreciated units in our codex. Their longer range helps balance out their limited mobility, and means they can dig into a nice spot of cover and generally stay there for the saves. They are suprisingly durable when in cover - not gonna win any prizes of course, but enough firepower to force a moral check (which, yes, will probably send 'em packing) will often be enough firepower to also take out an equal number of points of any of the other three options, particularly when you add in that they are the only unit in this list with the option to go to ground, which can buy them another round of shooting if it comes to it. But their big benefit is that they are cheap, cheap, cheap. Costing a buggy and a half, or a kan +10 or less then a single buzzsaw kopta, but with three times the firepower of any of those. Toss in a few ammo grots and they are more accurate then any of them as well.