Either of those will work, but from my experience, the Rock-it is one of the most cheaply built lifters out there. IMO the Covermate brands are the best, most durable well built lifters.
You need to have the spa in place (empty) FIRST, in order to install the slide under brackets. Simply pry up the corner of the spa with a small flat pry bar, and slide the bracket under (the spa MUST be empty of water).