Which way is more correct for dimensioning of the rectangular symmetrical feature?

Think in terms of tolerancing. Each individual dimension holds its respective geometry within specific tolerance limits. The left figure holds the right hand edges to a tighter tolerance than the left hand edges. Whereas the right hand figure holds the centerline of the whole geometry to a tight tolerance but allows both edges to vary even more due to their tolerances.

How you dimension it does not have a particular correct or preferred method. Each is equally correct. But what matters is how it is to be fabricated and then used that determines the proper dimensional order and tolerancing.

Also, you need to consider how you will actually measure the part to see if the geometry matches the given dimensions. In the case of the left hand figure it is quite easy to measure between the hard edges of the given geometry. In the case of the right hand figure you have dimensioned to a centerline floating in midair that has no hard geometry to measure to.

The most correct way to dimension a rectangular symmetrical feature is to use the following approach:

Locate the Centerline: Dimension the centerline of the feature relative to a relevant datum or reference point. This establishes the central location of the feature.

Dimension Overall Width/Height: Provide the overall width or height dimension of the rectangular feature. This defines the complete size of the feature.

Symmetry Symbol: Apply the symmetry geometric tolerance symbol to the centerline, referencing the appropriate datum plane. This ensures that the feature remains symmetrical within the specified tolerance.

Example:

Centerline Dimension: Locate the centerline of the rectangle 50mm from the left edge of the part.
Overall Dimension: Specify the overall width of the rectangle as 30mm.
Symmetry Tolerance: Apply a symmetry tolerance of 0.2mm to the centerline, referencing the top surface of the part as the datum plane.
Why this is the most correct approach:

Clarity: It clearly defines the location and size of the feature while explicitly controlling its symmetry.
Functionality: This method ensures that the feature is centered and symmetrical, which is often critical for proper fit, function, and aesthetics.
GD&T Compliance: It utilizes Geometric Dimensioning and Tolerancing (GD&T) principles to accurately communicate the design intent and manufacturing requirements.
Additional Considerations:

Datum Selection: Choose the appropriate datum or reference point based on the functional requirements and the overall design of the part.
Tolerance Value: The specific symmetry tolerance value will depend on the application and the desired level of precision.
Other Tolerances: In addition to symmetry, you may also need to specify other tolerances such as flatness, parallelism, or perpendicularity to fully control the feature's geometry.