Is it better for me to use a Canon 70D or a DJI Osmo Pocket 2 Gimbal camera inside the front seat of a car?

Depends upon the angle of view you need and the light level within the car.  Also, are you going to be on a smooth or very rough road?  Do you have a stabilized lens for your 70D?

I'd use the 70D if, and only if, the video capabilities allowed me to do what I needed to do, and I owned the lenses that I need, too. 

If both cameras produced the same level of video quality and the lens in both was okay for my needs, I'd go with the Pocket 2 because of the gimbal which will make for very smooth videos.

The Pocket 2 is a lot smaller, portable, easier to hand off to someone else to use, too. 

The 70D has an APS-C sensor, while the Pocket 2 has a very small 1/1.7" sensor.  If you're going to be using high-ISO (i.e. low light videos), you're going to get a lot more noise with the Pocket 2.

The depth of field on the Pocket 2 is going to be much greater because of the small sensor even though it has an f/1.8 lens.  Using an equivalent lens on the 70D would produce several stops less depth of field.  Now, this can be a plus when you need to get more depth of field, but a total negative when you want to intentionally blur out backgrounds.  I'm sure that you won't only ever use the camera when inside a car.  So what happens when you want to do some other kind of video with different needs?  The Pocket 2 with it's fixed 20mm (35mm equivalent) lens is a one-trick pony and extremely limiting.

It's much the same as your GoPro question - but we still don't know what you want to do.  Tell us what sort of shoot you're planning and we can make recommendations about equipment choices.

You should not hold a camera when you're driving. Both hands on the wheel, young ?