You just made half of the Feds' argument and half of the State's.
The Feds are arguing that the insurance mandate is necessary because without it, the rest of us will be forced to carry the costs of both treatment for the uninsured and premium rates for the unhealthy who choose to purchase insurance. Only with the uninsured, those who choose to be uninsured generally being young and healthy, can there be a feasible cost structure to make the rest of the Act workable.
The State is making a Tenth Amendment argument that the mandate of lack thereof, in fact the entirety of the law, is solely within the purview of the States and the Federal government has no power to enact or enforce it. Similar to your statement concerning auto insurance.
And of course, adults can always choose not to go to the doctor if they're ill or injured. Children have mandated medical exams and vaccinations to attend public or private schools or child care facilities and are required to submit proof of such even when homeschooled in at least some states, and failure to seek medical care when needed can be considered neglect, but there is no such requirement for adults. So strictly speaking, medical care is also a choice, not a necessity. Most things when you get right down to it are choices The alternatives may not be attractive, but there are always alternatives.
All factual statements but still not addressing the core issue.
Children have been required to get vaccinations etc. before attending school for a very long time now including well before either the federal or state governments got involved in that other than to urge parents to get it done. Parents paid for it or in most places local communities set up periodic free vaccination clinics. But all kids were required to go to school and all were required to be properly vaccinated before doing so and it got done. Without involvement of the federal government.
Children have always gotten sick and needed medical attention from time to time, and that was true before the the federal or state governments got involved in any way. And most parents took their kids to the doctor and, if they didn't have money to pay at the time, they paid it out a few dollars a week or month until they paid their bills. Many a time a local community took up collections to help out with an needed operation if the parents couldn't come up with the money. Sure some parents were irresponsible and didn't do right by their kids but that was always the exception rather than the rule. I think there are probably a higher percentage of parents now who don't do right by their kids.
Healthcare is no more essential to life, health, and happiness than is food, shelter, and clothing and it makes no more sense for the government to take over or control the entire healthcare industry any more than it makes sense for the government to take over and control all the other essentials of life.
The government should spend its energies setting up and enforcing anti trust and RICO laws and setting minimal standards of quality and safety for products and services and then let healthy competition in the free market make healthcare affordable as it does in all other essentials of life. We'll all be better off.
The question posed was whether the health insurance mandate was different from the auto insurance mandate enforced via civil and criminal penalty by the States. The straight factual answer is, it really isn't.
The straight factual answer is, it is very different for all the reasons already enumerated.
If MA wants to mandate its citizens must buy health insurance, fine. No one has to live there and states appear less limited in what they can do.
But the Federal gov't is different.