It depends on what you're trying to calculate. Both will give you acceleration, but the question is which acceleration you're calculating. Symbols are what names are to us humans, and just because two people are named John Doe does not mean that they are the same person. Likewise, if you use F=ma in this example you'll calculate the acceleration caused by gravity (often labeled g), while a=Δv/Δt will give you the deceleration (negative acceleration) of the rocket.

You need to look at the data you have and figure out what corresponds to what. Drawing a free-body diagram can help with this.