What is the difference between NSRectFill and NSBezierPath(rect).fill

I know I can fill a rect using NSRectFill(bounds). However I wanted to preserve transparency for PDF output and I discovered that I can do that only with NSBezierPath(rect: bounds).fill() What is the difference (behind the scenes) of those two?

func drawBackground() {
    CGContextSaveGState(currentContext)
    if (NSGraphicsContext.currentContextDrawingToScreen()) {
        NSColor(patternImage: checkerboardImage).set()
        NSRectFillUsingOperation(bounds, NSCompositingOperation.CompositeSourceOver)
    }
    NSColor.clearColor().setFill()
    //NSRectFill(bounds) //option 1
    NSBezierPath(rect: bounds).fill() // option 2
    CGContextRestoreGState(currentContext)
}

extension NSImage {

    static func checkerboardImageWithSize(size : CGFloat) -> NSImage {
        let fullRect = NSRect(x: 0, y: 0, width: size, height: size)
        let halfSize : CGFloat = size * 0.5;
        let upperSquareRect = NSRect(x: 0, y: 0, width: halfSize, height: halfSize);
        let bottomSquareRect = NSRect(x: halfSize, y: halfSize, width:halfSize, height: halfSize);
        let image = NSImage(size: NSSize(width: size, height: size))
        image.lockFocus()
        NSColor.whiteColor()
        NSRectFill(fullRect)
        NSColor(deviceWhite: 0.0, alpha:0.1).set()
        NSRectFill(upperSquareRect)
        NSRectFill(bottomSquareRect)
        image.unlockFocus()
        return image
    }
}

I'm mostly an iOS programmer and not very fluent these days over on the AppKit side of things, but my guess is that you're getting the wrong NSCompositingOperation. I see from the docs that NSRectFill uses NSCompositeCopy. Perhaps it would work better if you used NSRectFillUsingOperation, where you get to specify the compositing operation.