Okay so without straight knowing your team here's my suggestion
First their base stats
Raichu has higher speed and attack but worse bulk and special attack.
Raichu also has a better movepool (in my opinion) with access to similar moves as Ampharos and also Nasty Plot.
Since bulk doesn't matter that much in games, and the whole thing is to hit hard and hit fast, I'd choose Raichu. He outspeeds most of the NPC Pokemon, hits hard and can hit both physically and specially unlike ampharos who is limited to specially
Therefore I'd choose Raichu
Hope I helped and GL choosing!