A) Whoever attacks first wins :
Example Glasscannon mages at lv 6 in League of legends. Everything dies in 1 hit. <Annie> at lv 6 can blink and instant kill <le-blanc> in 1 button press, but <le-blanc> could have pressed the attack button before annie blinked, instantly killing her when she comes in range (after blink). So a skilled annie should have blinked after le-blanc pressed attack button on a minion.
Benefits:
1) prevention is important: Since everything dies in 1 hit, you know that if someone attacks first you will instantly die, so you watch spell ranges, skillshots, buy wards. Support champions become usefull because they can buy wards and grant vision of area.
2) Lucky players can go 1vs5 and get penta without losing 1 hp: example fed karthus/fiddlesticks/trynda/yi against nabs.
3) Class imbalance doesnt matter because everything dies in 1hit. This allows more class divercity. You can win even if you have inferior gear, or are an inferior player.
cons:
1) player skill relies only who jumps first on others.
2) battles can last 0 sec [ opponent dies in exactly 1 hit ]
3) melee champions are useless in group battles.
4) can cause rage, trolls, negative attitude.
note: blablabla you can buy armor, but i dont consider it as an option in my analysis in order to demonstrate what "whoever attacks first wins" playstyle means.
Another example is vannilla wow, even if battles lasts longer e.g 6 seconds you will be stunned 100% of your time and just die without being able to do a single thing [i forbid pvp trinket for this mode].
B) Tanky players like in wow cataclysm. Battles take 15-20 seconds to end.
Benefits:
1) The effect of a battles can change even if you get jumped. Skill plays a more important role here.
2) you can "play" before you die.
Cons:
1) nabs wont be able to beat skilled players.
2) class are too homogenized. Why should i play a druid when a warrior does the same job and without having to heal ? It seems the warrior is better, he wins in pvp without having to run away like a coward and he has a much easier playstyle. And the druid gets no rewards for playing a harder class.
3) if enemy has better gear, it just gives the illusion you have a "chance" to fight back. Why not instant die in 0sec (as mercifull death) to avoid watching the 15 second pvp fight.