Question

Ampharos or Magneton for a HGSS playthrough?

Answer 1

Reasons to use Ampharos:
Can be caught before Falkner or after Whitney, depending on whether you want easier battles against Falkner and Bugsy or more experience for the rest of your team
Higher HP
Learns some good coverage moves

Reasons to use Magneton:
More resistances (and not many opponents can hit its weaknesses)
2 STAB types
Higher speed
Learns a spammable STAB move (specifically flash cannon) earlier

Both of these are still mostly the same Pokemon, despite the minor differences I just mentioned. They're obtainable before Morty, evolve at around the same time, are electric type, and have high special attack. They also tend to be good in the exact same boss battles, so I don't think it would depend on the rest of your team, either. Just use whichever one you want. Though if you do use Ampharos, be sure to delay its evolution by 1 level to learn discharge earlier.
source
And here are some movesets for both of them.

Ampharos @ magnet/choice specs
EVs: 100 Spe
- discharge
- charge beam
- thunder wave/focus blast/signal beam
- thunder wave/focus blast/signal beam

Magneton @ magnet/choice specs
EVs: 100 Spe
- discharge
- flash cannon
- charge beam
- thunder wave

Answer 2

For a HGSS playthrough, I recommend Amphoras. Both have similar movesets, but Amphoras gets moves outside of electric, while Magneton only uses electric and steel moves. Amphoras also gets better move tutor moves, such as Outrage and Signal Beam. Amphoras has only one weakness to ground but not many resistances. Magneton has a really good defense coverage but is weak to fire, and severely weak to ground. Also, Magneton's stats aren't as good as Amphoras. Although, Magneton has a really good special attack.

Comments

Tutor moves are post-game in HGSS. Not many people in HGSS use ground moves, so Magneton's extra weaknesses aren't a big issue. Magneton also learns flash cannon much earlier than Ampharos learns any powerful and spammable STAB move.