They taste the same.
Whitener is less sticky. And cheaper. And you get more in a pack. So... that's me sold.
Winner: Sainsbury's Coffee Whitener
Ow! Ok! Ok!
...The Snob has very kindly and gently pointed out that this is a rubbish review, so let me start over.
I've been using Coffee-Mate for a long time because I can't use milk (intolerance) and I don't like the taste of soy milk in Coffee. This means that I'm very, very used to the taste of this ubiquitous Coffee creamer.
I found the Sainsbury's Whitener while browsing the grocery section on their website, and at first I dismissed it because it has more cow's milk content than Coffee-Mate. But the thought stuck with me. And as my intolerance goes in ebbs and flows, I finally figured I'd try the Whitener while I was having a good period.
It turns out there is no appreciable difference.
No, seriously. The imitation creamer is actually really good. It's well worth trying out if you don't like/want milk in your Coffee and you don't want to use a brand name product for whatever reason, be it cost or, y'know, consumer conscience.
I'm very happy with this creamer and I will keep using it. So my conclusion stands: Sainsbury's Whitener wins it for me.