Manectric is a common Pokemon obtainable fairly early at Route 10 and probably one of the better obtainable Electric types in the game. It's fast, it has a good Special Attack stat, a good Mega Evolution, and a decent movepool, though through level-up it's fairly shallow outside of Electric-type moves.
Its overall good stats and solid matchups against key gym battles (especially Winona and Wallace) means that Manectric is a very hard Pokemon to pass up. I greatly recommend it.
Pros:
- One of the best Electric types obtainable
- Solid movepool
- Learns Discharge fairly early (Lv. 30 in ORAS)
- Access to a Mega Evolution
- Fast with good stats (105 Sp. Atk and Speed)
- One of the best users of Overheat
Cons:
- Limited level up movepool outside of Electric type moves
Now onto Mightyena, and I'm just going to make it quick by saying it isn't great. Its stats won't cut it by later in the game, and its movepool is fairly lacking. It makes good filler, but by the time you catch the Box legendary, you can dump Mightyena in favor of it. In the earlygame it isn't terrible, it evolves early and its stats are fairly high by then, and it learns Crunch and Sucker Punch through Level up, but I can't recommend it later.
Pros:
- Evolves early with fairly high stats
- Gets some very good moves later through level up such as Crunch and Sucker Punch
- Good attack (90)
Cons:
- Stats are not great later in the game
- Movepool consists largely of Dark and Normal type moves
- Fairly slow (70 speed isn't great)
Who do I recommend? Well, if you'd like to, use both? Mightyena makes a solid Pokemon up to the 5th or 6th gym. Manectric is a fantastic Electric type that you'd be hard pressed to pass up, since there aren't many Electric types available, let alone one with a great Mega Evolution on top of that.