Actually, you are not restricted to either a 5-course prix fixe (each guest's choice of a structured menu from the a la carte menu) or a 7-course tasting menu (which is pre-set for all dining) at Del Posto if you are dining as a party of four or less. You can order a la carte, or choose the 5-/7-course options in the dining room or at the bar (which can now accommodate twelve diners). You are only limited to 5-/7-course menus if you are dining as a party of 5-12 people. The website does have an a la carte menu posted but it is not specifically titled as such.
I have also noticed a broad spectrum of attire at both Babbo and Del Posto. While the ambience at Del Posto suggests a more formal dining experience, I have seen jeans and clean sneakers in both restaurants. And often.