I can tell you all not sure about the axle width, spread axle trailer is always better than closed tandem. Tracks better, smoother ride, distributes weight better for less road wear. Not sure about using it for excuse to make skimpier frame, if so shame on the company's doing this. As a former truck driver with over 1m miles, spread axle should be standard on all trailers by now, the argument for increase tire wear only applies to incompetent operators trying to jackknife their overloaded trailer on rough concrete or uneven terrain (these low TT weights won't make a scratch on the tires).
I also think the Vibe (308HBS) is what we have decided to be our family's first buy. It has a great floor plan and exterior looks aerodynamic enough to hold up and pull well.