Is seat selection done only upon check-in, either online (24 hours before the flight), or at the airport (i.e., not when you book your flight)? Looking at JetBlue for IAD-LGB non-stop vs. Southwest for IAD-LAX (with a stop at MDW). We'd then drive to the Upland/Ontario area. Offhand, JetBlue looks better in this case. Flying directly to ONT on Southwest would cost more.