Ziggs or Kassadin

Kassadin isn't a mage, he's a Assassin.

Kassadin is more bursty, and has a ton of outplay potential in his kit. He can 100-0 squishies instantly if you use your combo properly, and can kite tanks really well.

Ziggs is one of the safest laners, and almost always farms up to glory, but lacks the 100-0 ability Kass has. Gotta whittle people down then finish them off. Also has a global ult.

Both are strong in their own ways, and fit into different comps.

Kassadin 100% Ziggs is too immobile, and is really relient on skill shots. Kassadin has a point and click silence interuption, and is extremely mobile.

Honestly, ziggs. Kassadin has suffered too many nerfs, and with his inability to do much unless super fed, he wouldnt be very consistent.

Ziggs is more consistent, and has a more stable laning phase, and power curve. He also provides good siegeing potential to a team, and has his ult to win his team teamfights

This completely changes based on both team compositions, one is an assassin with high mobility and low wave clear, while the other one has limited mobility and crazy wave clear. Ziggs is meant to be played when the game needs to be stalled, or you need your team to scale, Kassadin is to be played if an assassin is needed.

Two different champions and playstyles, its like comparing Diana to Vel Koz

Im a veigar main too, but i main him because of the crowd control he has, ziggs and kassadin have neither, however i'd recommend ziggs in that case.

Yes, that is a lackluster slowing spell, its good for zoning though, so cutting off routes in the jungle, or preventing tower dives, ziggs is all zone control, which makes him annoying to siege against, a lot like xerath or lux.