This is the same base with a bigger power supply. The CSL DD 8 runs the Boost Kit PSU for 8 Nm and scores higher on our rubric; the CSL DD 5 is the cheaper starting point at 5 Nm. Most reviewers point straight at the 8 Nm version, because the jump in detail and authority is bigger than the $80 gap implies.
So unless the budget is at its absolute limit, the CSL DD 8 is the buy - you are only $80 away from a noticeably stronger, more detailed base on the same chassis. The CSL DD 5 is fine as a starting point, but its 5 Nm is a floor you will likely want to lift with the Boost Kit anyway.