I see what you mean. If it's the 3rd chain we are focused on (which most people will and yet my years of mmo experience tells me that this may not always be the best for straight damage throughput *just a feeling*) then we should definitely look at the 2nd chain spells and see which is best. Now, keep in mind some do sound very similar. Be sure to look at EVERY detail and compare and contrast the 2. The damage out put may be greater than the other while the other could have a faster cast time. Likewise, one may be more mana efficient which could make sense to use it since the 3rd tier is what we are focusing on anyhow. So to save mana means more 3rd chain spells casts.
Also some attacks are cast immediately around as AoE requiring you to be in melee range versus some AoE that is casted directly into target from a distance. Some spells are a conal attack.
Also note the elemental build up and consummation. When you look, changing poises grants you a buff depending on the main stance you switch into. So if I wanted to let's say focus on a certain element for burst damage, I may want to figure out which element that is first off, then consider if it would benefit the buff from switching into a different poise I use for said element while also taking into account the amount of elemental power I have built up.
So to wrap up, it really depends on how you want to play your mage. And this may take a while for people to dig into to realize the true power of their classes in general. So for now. I wouldn't worry so much about it. Just have fun and try it all or whatever sounds good. I guarantee there will be people minning and maxing for top performance, so before the clones start coming out of the woodwork, be creative and enjoy.