Let me begin with this: I am not a Pilot, just an armchair GA Flight enthusiast who knows something on this subject.
In this case, the main difference between a dashcam and an action cam is that the saved recordings are dealt with differently. Both record to a Micro SD Card which comes in various sizes, and eventually the card will get filled up with the recordings. When this happens with an action cam the recording stops and you do not lose anything recorded up to that point, but it doesn't record any more from there on. When this happens with a dashcam, recording continues, but the new recording replaces some of the oldest recording on that card, so you lose the earliest parts, and keep the newest parts with the cam still recording. So for a card that holds an hour of recording on a two hour flight, an action cam will have the first hour saved but nothing from after that, while a dashcam will have the last hour of the flight saved but nothing from before that as the old video was written over by new video., which is called "loop" recording.
Cards come in various sizes but both kinds of cams will have differing upper limits on card sizes which will work with that particular cam. Almost all will work with cards which hold an hour or more- sometimes a lot more. Most General Aviation flights will be within the limits of card size, but a cross-country flight may exceed the card's limits. So for most flights, either kind of cam will work without losing any part of the flight.
Another difference is that dashcams are meant to be externally powered while action cams have an internal battery to power them so no external power is needed. Kind of like the difference between a table lamp and a rechargable flashlight. Action cams can also be externally powered so that you are not being limited by it's internal battery capacity. This external powering generally requires a 12V source like a car has, and is usually done through a cigarette lighter socket and an adapter you can get with the cam. Almost all of the General Aviation planes will have a 12V power source, but some use other voltages. You can ask anyone who owns the same plane or anyone who works on them about this and they wil know that answer.
Now here is the tricky part. The power adapter used with dashcams and action cams is usually "noisy" electrically, which might interfere with radio and instruments and we certainly do not want that happening. Action cams running on their internal battery are much "cleaner" electrically and are far less likely to give this kind of problem. And anything which is connected to the plane directly might require the installation work be done by a Certified Airplane mechanic which will be costly. An action cam used with it's internal battery requires no wiring in the plane- like a flashlight it works all alone wherever you put it- and it needs no Airplane Mechanic to install it or FAA certification to use it. This is why action cams are generally used in planes instead of dashcams.
I would highly recommend you speak with another General Aviation Pilot or Airplane Mechanic to get assistance in making your selection of a Cam. If they don't know the answers, they will know someone locally who does. If you can't get help locally, send a message to Simon at
AvWeb.com or to
AOPA I'm sure you'll get some friendly and highly qualified assistance with this from either of those sources.
Phil