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You probably know this, but I'd like to clarify just in case. You cannot just choose between 212 and 214 interchangeably. You have to choose between two entirely different combos:
1) NJP - B12 - 212 - Ice Slide. Because NJP bounces the opponent, you have to use 212 as 214 will not connect.
2) JPS - 214 - Ice Slide. Because JPS does not bounce the opponent, 214 will connect.
Between those two combos, the first will actually push the opponent 1 dash length farther than the second. 1 dash length is not a game changer, but if you're a perfectionist with your play you would always go with the first combo when the opportunity arises.
This is just something that comes with practice. Know that it is not up to chance. If done properly, this combo can be pulled off every single time. Go into training mode. Practice FFB, FFB, FFB like it's Stryker's gun and you're on the challenge with the 3 Shao Kahns.
Yes, it is the same off the 212 combo. In fact, the X-Ray is easier to hit off the 212 combo because you can just press it. You do not have to hold it like in the 214 combo.