The longer the lever on the ball mount the more force it will apply to the receiver. Archimedes stated it simply over 1,000 years ago - "Give me a long enough lever and I will move the world" so you would want the shortest overall length that will still drop the ball to an acceptable height.
As long as the drop adapter is rated for a 3,000 lb or greater load along with matches or exceeds the tongue weight of the dolly or anything else you might tow then you should be good. Extra capacity on the height adapter won't hurt aside from a few lbs extra weight but you are wise to limit leverage due to unnecessary additional length.
That Curt angled model pictured is re-enforced that way due to the lighter duty materials its made of and longer overall length so it has a lower tow rating than the straight one which likely has at least double the capacity if not more. The angled Curt has more capacity than needed in this case when used as a drop being rated at 5,000 lbs GTW and 500 lbs tongue hitch weight. While the shorter straight one may be rated at 10,000 to 12,000 lbs GTW and 1,200 lbs tongue weight. The straight drops will be around 9 inches long while the angled ones can be close to or greater than 12 inches long.
Listing of 8 inch drops:
For expert advice you could also just ask E-Trailers Techs.