Luigi can string together attacks a lot better than Mario can. And Luigi can "combo" heavy characters really easily.
They're about even. Watch the last stock.
Mario can follow better vs characters with better aerial mobility. Luigi can get into heavier characters well.
I have this as 60:40 Ike. Luigi simply can't get in and stay in vs Ike. His terrible speed and defensive options leave him powerless against an Ike that can space. He's also much easier to gimp than Mario because of how vulnerable he is during the entire recovery. His gimping ability is also a shell of Mario's.
Basically, Luigi has power and a good recovery. Both of those are taken away by the fact that Luigi has no defensive game to speak of, less than adequate approach options (lol, fireball), and an easily gimped recovery. If the Luigi's smart, he can get around the gimp pretty often by recovering high, but that still leaves two game-breakers.
Luigi also combos Ike pretty well...better than Mario can. Getting inside is going to be the main problem for Luigi throughout the entire match. Since Ike excels at keeping opponents to a distance...well...