I would definately say go with Square top it gives a lot more drive up top, I've only had mine on the 5.7 Taipan for 6 sails and swum 4 times but I was going faster each time. Your mast section is important if its too light you won't be able to rotate much or it will bend over backwards when you pull the main on and sideways if you dont have the main on enough. The design is really limited by your class rules on size and shape. Does your class include the mast in the sail area? If not you can get a stiffer section like an F18 which gives you the stiff mast and technically extra sail area. I know the Stingray sailors pick up about .4m2 doing that.
Put a camera on your boat when you first put the big head on and post it on you tube, I've kept my mates laughing for hours with my videos