Tremors been my main for a while now. after shock only for the most part. as far as when crystalline should be brought out, would that be when i feel like my air shaker wouldnt be a viable tool? or should i be using crystalline for other reasons? losing the air low is my biggest issues but there are matchups where i feel like i cant even bust it out.
First off, Crystalline has the better armored wakeup, even making your overhead fist wakeup better because the low exists. Against rushdown characters or just characters you can't zone, that's a big deal.
But yes, if you don't feel like you are getting use out of air shock, then there's no reason to use Aftershock. Characters with teleports for example, they blow up Metallic and variationless zoning and some can wreck air pound as well. Scorpion is a great example. His teleport will blow up your zoning, Aftershock in particular will have marginalized use out of the air pound because Scorpion can full screen full combo punish it. But as Crystal, you can armor up at long screen which allows you to be more aggressive long screen, as the armor can guarantee you a punish against a teleport even if you get hit during a move.
One thing Tremor has an abundance of is ways to deal with wakeups. Overhead rock, lava restand, aftershock can jump back into a low, and Crystal has one hit armor which is fantastic in certain situations.
This is where Crystal can really shine. His offense can dominate against characters with one hit armored wakeups, Tempest/Warlock/ect. In the case of Kung Lao, you can marginalize the teleport(allowing you to zone better) and his crazy good wakeup with the armor. While you can probably zone him better in Aftershock, I see Crystal as being better here because it makes you a monster in the corner and let's you get out more easily with the extra wakeup.
Now against a character like D'vorah(multi hit armored wakeup, mediocre counter zoning), those benefits aren't as useful as being able to keep the opponent out, which Aftershock can do with great success.
Crystal can also stay safer and punish better if you know what you are doing with the armor.
Kitana is an example of someone I would go Metallic for. Crystal can work in this matchup too but it won't excel like it would in other matchups. Air fan will punish the hell out of air ground pound, whereas Metallic can at worst challenge Kitana's zoning and make it really difficult for her to get in.
But generally I look at it from a zoning perspective. Can the air ground pound be punished? Will I get enough hits out of it to justify it over armor and armored wakeup from Crystal? Can I zone better in Metallic? Do I need to go Crystal to maximize my offensive opportunities?