Addendum:

1440 x 1080 HD source uses 1.3333 PAR, to achieve the same 1920 x 1080 SAR.

Your question about 1.2121 NTSC DV Widescreen is irrelevant, as it pertains to Standard Definition, and not High Definition video.

HD isn't NTSC, so anything relating to NTSC wouldn't apply.

The output format you want is 16:9. 1920x1080 is already this ratio, so the pixels need to be square to match it. If you set the ratio to 1.2121 then your image would be stretched horizontally and you've have 'short and fat' people on the screen. There's also a 1080 format that is 1440x1080, and the pixel aspect ratio needed to make this come out to 16:9 is 1.3333. Don't worry about the shape of the physical pixels on the display screen. The hardware will take care of the stretching for you automatically. All you need to worry about is making sure that the pixel aspect ratio makes the output frame size match the shape of the format.

"I thought that the square pixels setting was for computer display and television displays rectangles."

Actually, back in the day when that was more or less true, neither televisions nor computer displays had pixels. They had continuous analog scan lines and the visual information changed at regular intervals to provide horizontal resolution. The pixel shape assigned to the video data when it was digitized was arbitrary and not at all dependent on the type of display. 1.2121 was chosen for widescreen TV because it allowed the same number of pixels in the frame (720x480) as full screen at 0.9091. 1.0000 was chosen for computer displays because it made calculating graphics displays more straightforward.

However, this wasn't always the case. The Apple ][, which was probably one of the first graphics-capable computers most people encountered, had pixels slightly wider than they were tall ... on most displays. But not all displays were calibrated the same so the ratio varied from one monitor to the next, often by a substantial margin.

Thanks everybody for the answers.
Another question about the frame rate:
With the Template : HD1080-24p (1920x1080,23.976fps) , the frame rate defaults at 23,976(IVTC) and this seems to work fine.
In order to get the most "film look like" footage, I wonder if the Frame rate: 24,00 Film would be an option?
My TV does support 24p film mode (3:2 pull-down, so it automatically detects a film-based source originally encoded at 24 fFs and recreates each still frame.
My bluray burner supports 1080p, MovieFrame(24 Fs) and outputs 24 Fs. as well.
Thanks again,
Wouter Slagter

the FPS should, in general, match the footage. If the footage is 23.97, use that. If it's 24, use that. 23.97 is the same as 24, but drop frame, which most video uses.

my footage is from a AVCHD camera 1080/60i and I can't shoot in the 24 film mode but try to find the best workflow how to edit my 60i as 24p media without losing quality.

Wouter Slagter