Porpoising is a term used to describe the bow of a boat continually lifting up and down, a bit like nodding your head. It will probably only happen when the boat reaches a certain speed, usually near the top end for the boat (but not always)
When deciding about any purchase you have to ignore detrimental comments made by salesmen about competitors products, its in there interest to sow the seed of doubt. You have to look at the products and judge them on there merits. Test drive all of them if you have to but definately have a good test drive of the boat you intend to purchase, you wouldnt buy a car after driving it around the car park so why a boat. Try and find other owners of the same craft and ask their honest opinion, new owners are not as reliable (still in love) as owners who have had there boat a year or two so look for older boats for judgement.
Re delivery dates in my experience they are not usualy reliable. I purchased my boat, was given a delivery date that was put back once and on the day of collection was still being fitted out, but I did get it in the end
Martin