As promised, here are the results of my scrutiny of images 4(b) (BT878 cap card) and 6(a) (LG DVD Recorder).
Firstly, the obvious differences - the DVD recorder faithfully includes the nominal analogue blanking regions (the black parts to the left and right of the image). This means that the aspect ratio is more accurate. If we assume that the test card is supposed to be a perfect circle, by my reckoning 4(b) has an aspect ratio error of 4.8%, while in 6(a) it is only 1.2%. It is of course possible to correct this, either by using the BT tweaker, or by capturing 704 x 576 then adding the NAB borders in post.
You should also be able to spot that the BT cap has more saturated colours. I didn't make any attempt to calibrate colours during the processing (only luma levels), but again this is not a big deal as it's something easily tweaked.
The comparison gets really interesting when you look at the images through a software waveform monitor. The LG recorder exhibits less noise in the luma midrange - the corresponding bars on the monitor are tighter, while the BT image has more line-to-line variability (zoom into the blue colour bar on 4(b) and you can see faint horizontal lines).
The LG image has more noise in the low and high ends of the luma range. There is a reason for this however - it is capturing a greater luma range than the BT card. The BT card tends to clamp the black and white areas, while the LG recorder captures and stores "super-black" and "super-white" luma values. Indeed, it manages to correctly capture the super-black strip in the PLUGE test pattern that appears earlier on the LD.
Looking forward to trying out the SAA7231 capture card.