OK Spike, I'll jump in.
The flagship of PE's product line is the D1 firebox. It is used in the Super 27, the Spectrum, the Classic and the Fusion, as well as the "Pacific" insert. Having the same firebox and outer convection shell, all four models perform exactly the same: The difference is basically looks and price (the ash removal system with hidden drawer is standard on the Classic, Spectrum and Fusion models, but can be added to the Super 27 model for $115.00). We first burned a Super 27 on our showroom test flue some years ago, and it was the first Winter we were able to walk into a warm store every morning since we'd burned the original Woodstock Soapstone and Hearthstone Heritage models. The difference was, both soapstone models, although still putting out heat, were dead burned out and we had to start our morning fire from scratch. The Super 27 still had hot coals in it every morning, after a 12-hour burn. Since I work long hours and burn time is very important to me, I bought a Spectrum, and still heat my house with it today.
The Summit, Summit Classic and Summit Insert models came later, in an effort to offer a larger firebox and more output for larger homes. The now-legendary (on this forum, anyway) EBT technology was implemented to make the Summit hold a fire as long as the smaller D1 models, while still meeting EPA requirements. In our mild climate out here in the Pacific Northwest, we've learned to be careful about recommending the Summit models, as a Summit with a full firebox is a powerful beastie indeed.
The Vista, Vista Classic and Vista insert all share the same (smaller) firebox, and have out-performed every other "size small" woodburner we've ever carried, in both the sustained heat ouput and burn time categories.
One of PE's big advantages is, they're built in a robotics assembly plant. A mechanical "arm" goes all around each firebox, full-bead welding every seam. The result is, PE fireboxes are tighter than similar but spot-welded stoves (might account for the longer burn times, we think). Factor in the lifetime warranty, floating solid stainless secondary burn chamber and truly "stay clear" viewing windows, as well as the almost-fanatical devotion of PE owners, and PE products are an easy sell.