The Maxxus and the Sarena Bay were my final choices also. They are both great choices. We are going with the Maxxus for two main reasons. First we don't want two loungers, second the foot therapy was much better for all seats in the Maxxus.
While the Maxxus is a large tub there is really only direct jetted seating for four people. The other two seats are really just cool down seats. Six will easily fit in the Maxxus though.This is something to consider if you are going to have more then four in the tub on a regular basis. The SB has great jetted seating for six. However foot therapy is only available on the bench seats on the side.
I don't really understand your concern about Sundance quality. I read this board daily and Sundance seems to be held in high esteem by posters here. I don't think quality is an issue for either model.
In Tucson Az. where I am buying there is about 2k difference in price between the two. Be aware the SB will require crane delivery unless you have clearance for flat delivery. Delivery people will not flip the spa sideways because of the shape. Good luck, you will be happy with either tub.