I have a 12'x15' Marquee rectangular.
They do offer a lot of options on these. Sunforger is just a marketing name for some of the UV protecting canvas treatments they use. It's worth getting them with fire retardant and UV resistant treatments, so they hold up for a very long time. But something to keep in mind also is that some people use their tents heavily, leaving them up for several months each year. Having one for events only, I would suspect that even the cheaper canvas options would hold up well. I'm not sure which mine had, but I think it was one of Sungforger + flame retardant options (probably the $1200 one). We got ours used, so we got a pretty good deal on it.
The price you see on the price sheets for the various canvases is just for the canvas. The pole/rope-package is a separate price on that line.
They also give you a choice of trim/dags, height, and whether the walls will have slant to them. Slant isn't a big deal, though it gives you a little extra storage space around the edges that you won't be tripping over as much.
I recommend going with the 7' height on these, but 6' is just fine. The extra foot keeps you from having to duck through the openings.
In terms of packing them up in a car, the hardest part is the poles. Since I have a 6' bed inn my truck, and the poles are 7', I have to pack them diagonally. It has 14 vertical poles for the sides, and the center poles (two upright and a cross-beam) each are two-section poles, so those pieces are shorter. The canvas is bulky, but folds up and can be stuffed anywhere.
You can see some photos of my tent and my truck packing at the following links, but I don't think I took any good shots of just the tent in the truck bed by itself:
http://ed.toton.org/cgi-bin/gallery.pl?g=2006.pennsichttp://ed.toton.org/cgi-bin/gallery.pl?g=2008.pennsicThis shot shows the tent packed with a bunch of other stuff behind it:
Laid out:
Assembled: