Is it better to observe properties or redefine setters?

Say we have this interface CustomButton:

@interface CustomButton : UIButton
    @property (nonatomic, assign) CGFloat minWidth;
@end

With every change, minWidthwe want to place ours again CustomButton. As far as I know, we have two solutions:

Observed property value

// In -initWithFrame:
[self addObserver:self forKeyPath:@"minWidth" options:0 context:nil];

// In -observeValueForKeyPath:ofObject:change:context:
[self setNeedsLayout];

Override minWidthsetter

// In -setMinWidth:
_minWidth = minWidth; // Side note: it an ARC project
[self setNeedsLayout];

Which one is the right decision and why?