Ampharos:
Pros
- Has a decent level up movepool
- Has a ok TM learnset
- Decent stats all round except in speed
Cons
- Most of its good moves are learned late
- Low Speed
- Somewhat bad level up coverage
- Its stats are outclassed in every way by Magnezone, the only thing Ampharos has is better HP, no 4x weakness to ground moves, and a better movepool
Magnezone:
Pros
- Ok level up movepool
- Ok TM movepool
- Great all round stats, except in speed
- Great defensive typing
Cons
- All its good moves are learned late
- Low speed
- 4x weakness to ground
- Bad coverage
Overall: I would pick Magnezone, despite having equal pros and cons. Even though Magnezone's good moves are learned late, its decent attack means it can still use physical moves (Ampharos can though too, and better). It may struggle mid game because of relying on physical attacks, but its stats and usefulness late game make it better than Ampharos, in my opinion. I'm not saying Ampharos isn't good, I just think that Magnezone is better. Here is a moveset for Magnezone.
Magnezone @ Any
-Thunderbolt
-Flash Cannon
-Hidden Power/Tri Attack(if you didn't evolve right away)/Filler if its HP type is bad or overlaps
-Thunder Wave/Filler
I'm gonna put a moveset for Ampharos too, if you use it.
Ampharos @ Any
Thunderbolt
Hidden Power/Filler if HP typing is bad or weak
Thunder Wave
Volt Switch/Filler
I hope I've helped!