well, different cards, different use cases
obviously the manasustain and aoe are more usefull in lane while the stun is better in fights, but if your opponents stand tights, the aoe from red shouldn't be underestimated, same for it's slow, and once you find yourself low on mana, you really appriciate haveing that blue up your sleeve.
Not saying the 2 sec stun isn't more powerfull most of the time, but the other cards have some clear usecases aswell.
Though changeing the damage a bit would help, right now the stun has by far the lowest base damage, but the issue is that most of the damage comes from ratios, so that hardly matters. If the ratios also would differ between the cards, we could really get a choice between high damage on blue, good aoe on red or low damage but stun on yellow.
However, i dislike the stuncard for a different reason: global TP and a point n click stun are a sick combination that hold the rest of his kit back as his numbers can't be allowed to really be good.(similar issue with current ryze)