[*]Wire in, or battery? I may decide to wire in if it lets me record anytime I'm in the car.
Hard-wiring your dashcam is easy to do, and offers the convenience of starting/stopping the camera with your car (providing an appropriate fuse is tapped). I'd definitely recommend this.
The 12V socket is permanently powered on in some makes of cars, so it might not be the ideal source in such a case. Moreover, having it plugged in runs the risk of someone unplugging it accidentally.
That said, consider if you need to enable parking mode (detect bumps on the car to trigger recording), especially if you park in the street. If so, you'd need one with a battery.
Also, consider if you're going to be parking without cover under the hot sun - in such cases its safer to skip a battery and look for a camera with a beefy capacitor instead (the cap stores enough power to power-off the dashcam cleanly).
[*]After an "event", can a dashcam save the video easily and quickly (to smartphone, "other", etc), so that removal of the SD card (or the camera) won't leave me without video proof?[/list]
You can yank the microSD card from the camera and either plug it into your phone to view the video contents, or you can use a microSD to micro-USB reader (like this
) so you don't have to open up your phone to plug in the microSD card from the camera.
There are models of dashcam (e.g. Transcend) that make the dashcam a Wi-Fi hotspot, so you can connect to it to view videos or manage its contents. Keep this as a 'nice-to-have' when shopping.
When I was shopping for dashcams, I found the site https://dashboardcamerareviews.com/