Mega Hercross: Speed+Attack or Def+Sp Def Ev training?

Question

Those are my main questioning ev training atm, but feel free to suggest others.

As you probably already know heracross's max speed is 273 which isn't bad but could be beat since most people build speed ev's. Not many people build def/sp. def anymore and worry more on the first(speed+attack(or sp. attack)) so I figured def/sp def ev training would be good with that. I am just kind of torn because both are pretty valuable depending on the situation.
My attacks are:
Bullet seed
Rock Blast
Pin Missile
???
My 4th one is up in the air, I was thinking Rest which would be beneficial with my def/sp def build, but I could also go earthquake or aerial ace for more damage with other types of Pokemon.

With 515 attack and 273 speed and assuming I do go first, could I 1-hit almost anyone that I was at neutral effect at?(like not super effective but also not, not effective either) Because those moves do 125 damage each time, with his ability.

Or should I just 126 def and 126 sp def, and put 252 into speed?

Answer 1

I'd go with Speed and Attack Evs. Mega Heracross is more of a Physical Sweeper. Here's a Good Moveset I recommend:

Heracross-Mega @ Herracronite
Ability: Skill Link
Evs: 252 Atk / 252 Spe / 4 Def
Jolly Nature (+Spe, -SpA)

Bullet Seed
Rock Blast
Pin Missle
Arm Thrust/Close Combat

Your 4th Move Can be Arm Thrust to utilize Skill Link or Close Combat For all-out power. Both of these Moves are STAB.

Hope this helps :)

Answer 2

I know a "Best answer" has already been chosen, but I figured I would add another option. Mega Heracross is indeed mostly used as an all-out speed/attack Pokémon, but if you are pretty sure you'll be going last anyway (fast meta), you can invest into Attack and Hit Points. (Full HP investment leads to some surprises for the opponent sometimes).

A defensive build for Heracross is a little outdated, but of course if you felt like it you could, using Guts and Rest/Sleep Talk. (not recommended)