Those aren't mutually exclusive. You need both.
FWIW, if you liked your Synthe, why not get a Synthe MIPS? For that matter, if one Giro fits your head well, you should be able to wear any helmet in the Giro line with a similar fit. Other brands may or may not fit - you have to try them to see.

It doesn’t make me wonder. Plenty of people survived accidents in old cars, too… But I still prefer to have features like safety belts, airbags, active accident avoidance systems, etc.

No doubt MIPS is likely safer. I will though say I have wondered if there's any level of negation of the improved safety if one wears a skull cap, beanie, balaclava, etc underneath. Was a couple years ago, but the product safety guy at Bell said that they have to officially advise against this as this has never been researched (nonetheless he said that he himself and other colleagues do it anyway). A similar enquiry to MIPS themselves never received a response. Typically I wear one of those UPF caps for some sun protection.

I would expect just the opposite. Mips basically allows the head to rotate within the helmet, reducing rotational forces in a crash. If anything, I would think an extra layer between head and helmet would simply be more of the same, albeit not in a controlled fashion. Consider - they don't make specific recommendations for hair styles, and I would expect a full head of hair to act somewhat like a cap relative to a bald head.

Seems like it'd be an easy enough hypothesis to test. Maybe one of the helmet manufacturers or independent labs will do that. I rarely ride without a cycling cap under my helmet, so I'd be pretty interested in an actual test.

BB

Well, no…MIPS allows the helmet to rotate around the head.

Well that makes ALL the difference!!

Well, no. The MIPS layer goes between the helmet strapping and the outer polycarbonate and foam layers of the helmet, whereas your cycling cap, beanie, or bouffant goes between the helmet strapping and you head.

If you’re going to think about how the system works, it seems worthwhile to think about it correctly. Per MIPS:

Think about a fall on a slippery slope: If you fall and hit your head (or helmet), you will most likely continue in the same direction as before the fall. Alternatively, consider a similar fall on high-friction asphalt– a very different scenario. When your helmet hits the ground, there will most likely be a sudden stop and a rotation of the helmet. Mips® safety system intends to redirect that energy away from the head.

In summary, the difference between a helmet with and without a Mips® safety system is that a Mips® safety system allows the helmet to move 10-15mm relative to the head on certain angled impacts.

There’s lots of good info on the MIPS site: https://mipsprotection.com/science-technology/

Now, if you are in a tight spin (like an out of control airplane) when you hit the stationary road, your head will be spinning in a stationary helmet!

That’s incorrect. In the attached picture, you can clearly see the MIPS layer is between the EPS foam/straps and the head.

OK, I stand corrected. [Thinking about simulating bbbean's hypothesis by doing my hair with vaseline before my next ride.]

It still allows a similar effect. The point of mips is to dampen rotational forces in a crash. I think it is at least conceivable that a slippery layer (hair and/or hat) could provide a less controlled benefit. I think it would be worth testing.Then this wouldn't have to be a matter of conjecture.

But kudos to you for objecting to my general reference with a more specific reference. I'm sure we're all better for it.

Oh I agree! but when one spherical object is rotating relative to an enclosed sphere, it really doesn't matter whether you say the head rotated relative to the helmet or the helmet rotated relative to the head. The point was that a limited slip in the system dampened rotational forces.

For the record, that research will be useless without pictures and/or video.

This is truly a once in a lifetime opportunity. If we're going to do right, we need, at a minimum, pre and post exposure neuropsych and balance testing, 3 and 7T MRI, serial serum and CSF proteomics (with SIMOA) and gene expression, and, in the worst case, tissue pathology.

It totally matters because of the force vectors. The entire point is to redirect impact force into moving the helmet and minimizing that force moving the head.

Again, reading the MIPS site I linked above is quite instructive and helpful.

I also stand corrected, actually! Poking around the MIPS site, I read this:

In addition to the shell and foam structure, helmets with a Mips® safety system have a sliding layer added to the helmet structure. This low-friction layer can be installed in the helmet at different distances from the head. Most helmets with a Mips® safety system have the sliding layer positioned closest to the head, allowing a sliding movement of 10-15mm on certain angled impacts, intended to help reduce rotational force to the head. Alternatively, some helmets have a Mips® safety system positioned within the EPS foam, between the foam and the outer shell, or even outside the shell.

So it is not absolutely or necessarily true that the MIPS layer contacts the head. I’d guess a bike helmet which uses a hardshell, not a microshell molded or bonded to the EPS as is typical, could have the EPS layer between those two elements. I haven’t seen a hardshell bicycle helmet in decades, but maybe they’re out there somewhere. More likely, I think that placement is used for other helmets in other sports, like horse riding, where hardshells are used.

Ooh, ooh, can we also do that "Person. Woman. Man. Camera. TV" test, too?

But of course!

Maybe in a couple of decades, after my son has made it through college and I am somewhat senile. But then I may not have much hair left to vaseline.

Either way, you're a sport!

MIPS is a must have and not a problem as nearly all the helmets being sold for road or trail use have MIPS at this point in time. For me the issue has been with the neck straps and most helmets are not comfortable for extended use. I have found helmets from Lazer, Bell, and Scott that have both MIPS protection and comfortable straps (if adjusted for best fit).
MIPS is designed to protect the brain stem and this would not be a trivial injury. Only downside is fewer organ donations by fit cyclists.

Actually, it's somewhat interesting. If you visit Wiggle/CRC website, and redo the preferences to pretend you're a UK resident and shopping for UK shipping, you get a pretty different picture of what is selling well. Not sure if this link will work for most, or if you'll automatically get redirected to a US webpage: https://www.wiggle.co.uk/cycle/helmets

Another thread that went sideways.

FIT for sure, then the rest. If you don't like the feel of a helmet, don't buy.