As one of those who hold a small part the blame for causing some of the threads on this board to degenerate, I can honestly say that I can't see this ending in a way that leaves Spoon without a migrane and wondering if that picture of dawn in the onsen was a mistake.
But back on topic: It depends on what kind of 'pin-up' it is. If it's a standard 'Don't mind if my mum sees it up', say a motivational poster, Crystal would be the best in most cases, with Isa being the best for more action focused ones with many red lights and explosions.
Now for the more....risque ones, Crystal with her cutsey appearance would be better situated something tastefull and modest; a scene with her in pyjamas or something along those lines. Isa would just have her smirking face and a caption that reads: Did you really think I'd lower myself and allow my form be something for you worthless humans to gaze at in lust?"
Point being, it's all in your personal taste. Me, I'd rather have a poster of Misuzu's mecha collection.