Santa Monica Airport sits about four miles from most of the city and Van Nuys sits about twenty, so the question deserves a straight answer rather than a sales one.
Start with the part that is a matter of public record. Under a 2017 consent decree between the City of Santa Monica and the FAA, the runway at KSMO was shortened to 3,500 feet, and the airport is publicly reported to be scheduled for closure after 2028. That is not a comment on anyone operating there. There are good instructors and good operators on that field, and they will be flying somewhere else afterwards. It matters to you because a private pilot certificate takes most people nine to eighteen months, an instrument rating usually follows it, and the aircraft you rent after that has to live somewhere. Choosing where to start a multi-year path is a decision worth making with a calendar in front of you rather than a map.
The second reason has nothing to do with any of that. Van Nuys is a Class D field sitting under the LAX Class B shelf and one of the busiest general aviation airports in the world. You talk to a control tower from the first lesson. Not in month four, not after you change fields. Ground, tower, pattern entries, sequencing behind traffic that is faster than you, all of it becomes ordinary while you are still learning to land. Pilots who learn at quiet airports get that education later, usually in a hurry, and usually somewhere they would rather not be learning it.
Now the trade-off, because there is one. Van Nuys is busy. Some mornings you will hold short behind two jets and a helicopter, and that is time on the Hobbs you would not burn at a sleepy field. We think radio fluency is worth it. If you disagree, that is a defensible position, but make it knowingly.
One more thing that is not glamorous and decides more outcomes than either: we own our aircraft and we have eleven instructors. Schools with three airplanes and two instructors cancel. A student who loses two lessons a month to somebody else's scheduling adds close to a year.