Serrano won't impress you with selection or decor, but what it will do is let the food do all the talking.
As a solid affordable lunch option, I've been to Serrano almost once a week for the past 3-4 years.
If I were to choose between breast and thigh, I break it down to how hungry I am. If I need more sustenance breast is the way to go, if flavor is what you're after thigh is the choice to make.
That being said, I'll throw a curveball in all that and say that my favorite is the pork sandwich. Yes, the pork sandwich from a place known for rotisserie chicken is my go to choice.
Maybe it's my Filipino heritage clouding my judgement, but the juiciness of the meat with the amazing chicharon-like crunch of the crackling skin is just perfection.
As other have said, no matter what sandwich you take, ask for it "All Dressed" with the chicken drippings and the spicy sauce (not overly spicy, I usually supplement my sandwich with some Valentina Black Label hot sauce of my own)