Ah, Seurat fit under the banner of impressionism--yes, his pointillism was used to express the subtleties of light variation--while Lichtenstein was a Pop artist. These are confining terms, but they do convey the feeling of the work of these two artists, and hence the differentiation between the two.
As the answerer above me has pointed out, Lichtenstein had, like Warhol, a commercial background, and this influence informed his work; he was working in manner most comfortable to him and most in tune with what he was trying to express. Even Pop Art expressed the modern taste, the "pop sensibility" of a generation.
In its own right, Seurat's work expressed the modern ideas of his time, the conveyance of subtlety of emotion, rendered through shading and softness of light. Seurat used pointillism as his method, but the goals of his work were quite in tune with his Impressionist peers, the modern artists of his day.
With so much in common then, the differences looked at are to be found in simple technique. Lichtenstein's dots can hardly be said to have the same feel and convey the same gravity as Seurat's pointillism. The difference between the two is one of magnitude and weight---Lichtenstein's work is much heavier on the eye, and does not seek subtlety.
One might say that the density of impact, although not the effectiveness, is the measure of difference between the two.
This answer has been brought to you courtesy of Jack, Fine Arts degree, University of Western Sydney