Align to the left
Based on your screen captures, you can see how the center aligned version requires a "jump" for the user, causing a cognitive load and momentary interruption of flow, which brings no benefit at all and is quite annoying.
There are 3 basic reading patterns: Gutenberg, Z-shape and F-shape, plus some sub-patterns from these main ones. There are others, but these are the main ones, based on many studies on how users read a page. If you check your center aligned version, it doesn't fit with any of the common reading patterns and how users visualize information. Simply put: If you follow the logical flow, you'll see that the line of content (left, where you find the title and the controls on bottom) is interrupted with a blank space.
This applies in your case because all remaining elements are aligned to the left. If they were aligned to center, you could have the form in the middle, because they would be in the same line.
An extra tip:
To increase ease of visualization and logical flow, try removing all the blank space that holds no information. Simply make the container box fit the width of the form and it will work just perfect