I'm a Schwartz guy. Even when I was working as a professor in Chicago, I would pay homage to the place via a picture of three Schwartz sandwiches (instead of a picture of myself) on my professor homepage. I still love Schwartz.
However, Lester's is fabulous as well. In fact, I may start going there more often for three reasons: (1) the quality is comparable - though I marginally prefer Schwartz, (2) they offer a larger menu of options - which is irrelevant to my tummy but relevant to those of my less rational friends, and (3) the place is more quiet - it truly has felt like a neighborhood haunt when I've gone.
To sum up, if I have smoke meat crazed friends in town and I want to maximally impress them, I'll go to Schwartz. Off peak hours and alone? Dido. Any other scenario? I may very well go w/ Lester's!