I use the motor that came with my Weber brand rotisserie. It is stamped Weber on the front plastic cover of the motor but is stamped Taiwan on the back metal case. I purchased the rotisserie ring and motor used about 5 years ago. The motor probably has over 100 hours of use on it now by me. I spin several turkeys each year. On Thanksgiving I always spin a 20 lb or larger bird. The motor is rated to loads up to 20 lbs but I spun a 24 lb bird a couple of years back. It handled it no problem.
Your Cajun Bandit motor will handle spinning a 20 lb turkey no problem Once you spit the turkey, the most important thing is to test spin your bird on the rotisserie prior to adding lit charcoal. That way you can adjust the bird if it is to far out of balance. Heavily unbalance loads will burn out rotisserie motors.
The One Grill HD rotisserie motor is designed for loads up to 85 lbs. It is more for spinning a whole lamb, pig or side of beef. IMHO, it is not needed for a 20 lb turkey. If it gives you piece of mind having and using the One Grill motor, go for it.
This was last years Thanksgiving turkey (19.25 lbs). Note the charcoal basket placements with the turkey somewhat offset. It puts the legs and thighs closer to the heat and the breast meat further away from direct heat. Helps keep the breast meat from drying out.