Do your PhD fast or strong? I am in the middle of my PhD and have some strategic decisions to make. I have the opportunity to finish my PhD more rapid, although it would compromise some perspectives/research, it will still (probably) be accepted. 

Alternatively, I can use my last year of PhD to research more to deliver a better overall PhD package, but then I would have to delay postdoc (and better pay). 

My supervisor's concern is that my PhD delivery is important for future references, so it would be in my(ours) benefit to make it as strong as possible. 

Is his suggestion valid? Would a stronger PhD delivery potentially be better on a "CV" than one year of postdoctoral research?
I think the quality of your research depends a lot on the quality of your environment. If you publish a few good papers, you have a higher chance to find a postdoc in a strong group and publish even more good papers, which will improve the chance to find a faculty position. No one will ask how long your PhD took. If you get lucky with your first postdoc, in the sense that you keep publishing high quality work with them, you may not have to do a second, or you would get a very strong second position and then obtain a faculty position.

On the other hand, if your intention is to eventually find a job in industry, or you already have some great opportunity, finishing early is a good idea. The extra-year would only give you more time to do a proper job search in this case.