Hard to say, you can’t go wrong with either of them, Iceman’s got fantastic utility and X-23’s got amazing bleed damage plus a nice regeneration. Just depends on what you need right now and of course who you enjoy playing.
Iceman is probably better just because he's not as situational as X-23, who is easily countered by bleed immune champs. Iceman is more of a "universal counter", as he can be used as a counterpick to anyone with a high damage output, evade, high health, bleed, poison, incinerate, and power gain (because ice armor will reduce the damage from specials).
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GP/IM or SL/X23