I have a GP-5W. I use a peak visor and no faceshield most of the time, great ventilation and easy to set up an intercom. Not a fan of the decreased protection of a half helmet, especially since I believe none of them are HANS compatible. Even if you don't plan on a head and neck restraint it's nice to have it available.
The yet to be released GP-6 is supposedly a very expensive hat, over $2000, the carbon version over $4K. But the new Snell 2010 which supposedly tests with HANS devices in place, won't be released until Oct. 2010, so you might consider waiting to either get a deal or a helmet with the newest safety rating.