I mainly use Gamblin and Winsor & Newton oil paints (I also use some Rembrandt, but the quality is noticeably different). I recently saw a Discovery channel special on the making of Gamblin paints, and it got me wondering if Winsor & Newton manufactures paint the exact same way. Luckily, I found on YouTube an episode of "How it's Made" (another Discovery channel show) featuring the Winsor & Newton process. After watching both, it is clear that although the process of making paint is the same, it seems that Winsor & Newton focuses much more on exactness, quality control, and lab testing. This is probably tied closely with the fact that Winsor & Newton is a much larger company and has a reputation as the "best" oil paint to keep up. From a personal, hands-on point of view, I like each brand for their own qualities. Gamblin has a generally fantastic consistency and transparency, but Winsor & Newton's colors seem purer while the consistency is a little thicker with less transparent in some cases (which may be tied to the actual pigment, not the manufacturing process).
To really study the options and differences between these brands, I've ordered their hand-painted color charts (which I honestly didn't know existed). This way I can put the swatches side-by-side and make sure I'm choosing the right brand for the right color.
For the Winsor & Newton click here
And for Gamblin, go here and at the bottom you can send an email requesting one.