They may arrive at the same destination, but two passengers on the same flight can have strikingly different travel experiences.One traveler breezes through a priority security lane and heads straight to an invite-only lounge for craft cocktails and a chef-prepared meal before boarding early. A flight attendant offering a glass of champagne and a warm hand towel welcomes the passenger to a spacious seat at the front of the plane.The other traveler stands in a line at every step — security screening, a café selling $16 sandwiches, a crowded gate — then boards with one of the final groups, hoping there’s still room for a carry-on in the overhead bin before folding into a cramped middle seat. After the cabin lights dim, sleep comes in fragments, and a travel pillow does little to ease a stiff neck.The contrasting journeys are no accident. Since the COVID-19 pandemic, the largest U.S. airlines have pulled out all the stops to court premium passengers who are willing to pay for comfort, convenience and exclusivity. Budget-conscious travelers may notice a widening gap between the back of the plane and up front as the carriers increasingly build their businesses around selling first-class, business-class and premium-economy seats.