I have a 24' Pace Shadow, which I am very happy with. It has a bit of a "beavertail" at the back to lower the ramp height - ramp length is about 7' with an 16" flip out extension - deck height is about 18" and my '06 C4S loads with no problem. If you have a stock car, you will be able to load with no problems - if you are lowered and/or have low hanging front aero, there may be issues.....
If you can figure out the approach angle of your car, basic trig will tell you what ramp length you will need for a specific deck height. The beavertail helps to not only lower the deck height, it reduces the angle of the ramp to the deck - this helps out to keep low points under the car inside of the wheelbase from dragging.
I am using e-track with Mac's wheel block straps (
http://www.macscustomtiedowns.com/product/280/WheelNets), and have been very happy with the system - the wheels are held securly to the deck of the trailer, with no "unnatural" forces applied to the suspension - I just don't like putting straps around the wheel spokes and trying to pull everything apart with the tie downs - just my $0.02 worth.
If you ever want to drive up to Idyllwild (and why woulden't you want to drive your Porsche to Idyllwild), let me know, and we can try loading your car on my trailer.
Hope that this helps.
Phil