Both cameras work well outdoors and have little difference in terms of resolution sets and accuracy. The best depth resolution you can get on both cameras is 1280x720.
The main distinguishing points relevant to your project are:
1. The minimum depth range of the D415 is 0.3 m and the minimum of the D435 is 0.2 m.
2. D435 has a wider field of view and so will find it easier to keep the driver's head in its view.
3. D435 is better at tracking moving objects because of a faster 'global shutter'
Edit: I should add that you will need to combine the camera's SDK 2.0 with other software platforms such as OpenCV, as SDK 2.0 does not have face landmark tracking built into it like the old RealSense SDKs for the R200 did.
I think one additional item to keep in mind for such applications may be that most users report D415 to be giving better performance than D435 for short range, and for smaller objects. What do you think?
I don't have D415, so I have so far not made any comparison with D435, but almost every user who has compared the two, suggest D415 to be much better for short range.
Another important point, though both D435 & D415 have 1280x720 depth resolution. The resolution of IR camera is 1920x1080 on D415, but only 1280x800 on D435.
Note that Depth is computed by D4 Vision processor, based on the IR Camera imaging. So input to D4 processor is very different in case of D435 vs D415.
So far I have assumed that this difference is what leads to better short range resolution on D415. What do you think?
The resolution chart for the two cameras gives an IR resolution of 1280x720 for both cameras, In Calibration mode though, the D435 has 1920x1080 instead of the D415's 1280x720 though.
I'm wary of putting too much emphasis on the D415 / D435 image quality issue. Though feedback strongly suggests that the depth image on D415 is better, it is easy to become automatically negative about the D435 without considering its merits. A doom-based viewpoint can become a self-fulfilling prophecy.
Yes it is true, the D15 excels at scanning smaller objects.
You can use Visual Presets to change the balance of the camera's settings, so you can get more detail at the cost of lower accuracy, or higher accuracy at the cost of lower detail.
Like with the R200 / SR300, the D415 and D435 will have areas where one is the better choice for a certain application than others. For example, the R200 could scan larger objects and full human bodies, but had the drawback of lacking the hand joint tracking that the SR300 supported. Over time it will become clearer what the specific strengths and weaknesses of rthe D415 and D435 are compared to each other.
The D415 excels at scanning small, static objects.
The D435 excels at scanning larger, moving objects and has less blind-spots in its vision because of its wider field of view.