Jolteon vs. Ampharos

Higher attack and access to Thunder Shock which is superior to Charge Beam and Volt Switch as well as Thunder Bolt and Discharge. This is offensively though. Ampharos is a better defender

As said, it's the moves available. If Ampharos gets a better quick move we will have to take him seriously. But with Zapados a few days away I'm not sure that will matter for either of them anymore.

I have an Ampharos with Volt Switch and Zap Cannon. I prefer it on attack over Jolteon. Zap Cannon is so energy efficient that it nearly completely makes up for the quick move. A full cycle of 4 Volt Switch and 1 Zap Cannon takes 12.9 seconds and deals 220 damage. On Jolteon, a full cycle of 12 Thunder Shock and 2 Thunderbolt also deals 220 damage, but takes only 12.2 seconds. These measly 0.7 seconds and a 22 higher ATK are all the advantage Jolteon has. In my book this does not compensate for the higher sturdiness of Ampharos. Of course, Ampharos moves are so slow, it is nearly impossible to dodge. So the main reason is that I like his attack animations better. :-)

I have TS/TB and TS/DC Jolteon, they are both great. Jolteon also has all STAB moves.
Don't have Ampharos yet, because his candies are so hard to get. Good luck finding candies to power one up.