RussR
En-Route
I would say that it's about equal, because FAA requirements and insurance requirements each fall into the category of "necessary but not sufficient."
I think that it really depends on which end of the scale of aircraft type you're talking about.
For small aircraft and large aircraft, the insurance and FAA requirements are the most similar. For example, buy a 172 and the insurer may not require anything different than the FAA. Similarly, buy a Citation and the FAA requirements will be largely in line with the insurer (initial training, endorsements, and annual recurrent training), with the likely addition of "mentor pilot" time.
But it's in the vast middle that insurance requirements are usually far more stringent than the FAA requirements. Heck, I was denied as a multi-thousand hour CFI one time for a G36 Bonanza because although I had plenty of A36 time and plenty of G1000 time, I didn't have any "G36" time. Denied unless I got 25 hours in the type. Or, as discussed upthread, I could go buy a TBM as a new Private Pilot with zero additional training other than a few endorsements (and those could be in any other airplane) and fly it perfectly legally. But to get insured? That would likely require initial and recurrent training, many hours with a mentor pilot, etc, if I could even get insurance at all.
In my opinion, the insurance industry runs the show for the middle range of aircraft - I'd say the "anything retractable up to 12,500 pound turboprop" range. I'm not saying that's necessarily a bad thing, as the FAA requirements are pretty light in this area.
Consider that it's perfectly legal for you to get your Private and Instrument in a 172, fly a 182RG a couple hours for your high performance and complex endorsement, do a quick high altitude endorsement in a 210, take an online course for RVSM, and then fly a brand new TBM off the lot at 300 knots and FL300 with no additional training and only 100 hours total time.