my opinion : choose amphy. Yes, rai does have better spd. & atk. but excuse me Sempiternus, but I think amphy's moveset is better. Nothing so great was added to raichu from pika but grass knot , nasty plot for me is a wasted move slot. A steelix could come in and equake raichu while it sets up and raichu is down to only a blobbish 55 base defense . Amphy could survive it and switch out defense doesn't matter a lot in games, I to like to OHKO poke like MY AMPHY does. My moveset for amphy would be signal beam, charge , discharge and thunder. Signal beam is for the confusion effect,charge is for increasing sp.atk of electric moves while raising your sp.def for safety of survival . Discharge is for that 30% paralyze chance which can help your speed a bit and get stab which gives you 120 power including stab effect. Thunder is for extreme sweeping capabilities . Discharge , paralyze , charge then thunder is a good sweeper set. Plus amphy can support all starters. Chesnaught by covering that flying weakness. Greninja by doing some huge amount of damage or OHKO because fighting types ( one of Greninja's weaknesses ) don't have the best sp.def so amphy could take a hit and hit harder. And Delphox by covering that water weakness.Not to mention that amphy has an almost like 99.99% godly sp.atk and poor rai would have stare in awe on how amphy takes that hit like a boss and does the discharge, paralyze , charge & thunder combination and a little signal beam confusion for a little more damage.Just telling, speed is not the most essential stat to win battles , in my pearl game , I chose piplup and raised empoleon's level to Lv. 100 . Empoleon's speed is not high like amphy's but still has huge sweeping power.Only empoleon was the poke I used to beat Cynthia despite its not so good speed. Not to mention that some people here take multiple times to beat her. Empoleon wasn't even Lv. 100 when we beat her, but he was over leveled . He was Lv. 86. Amphy is a electric type and FYI alder's and iris' Pokemon aren't walling or tanking based. So amphy can be useful . A very long answer but at least it has some points.