I'm not an expert in the Sparco line, but here are the 3 main differences that I know of between the R505 and the R700:
- R700 has adjustable lumbar support
- R700 has adjustable thigh support
- Material choices
The R700A uses the Alcantara (swede) material. I cannot speak to the R505 material, but I can say that the R700 is really nice.
The reason I am looking to switch is that I'm actually a bit too tall for this seat in my car. Eric made a custom set of seat mounts which dropped the seat as low as possible without cutting out the floor, and even still, I'm a little too close to the roof for comfort. So if you are 5'8, I would think these would be a much better fit for you.
I wouldn't necessarily have paid the normal price for these seats over the R505, but I got a heck of a deal on the pair, so I made the purchase. I purchased them slightly used, but due to damage in shipping, I received replacements that are brand new. We have mocked them into my car, but that's it. So I could make a good deal on them.
Also, whichever brand and model that you choose, you need to add the price for the sliders to the seats (about $100 - 125 per seat). Then you also need to get some form of mount to the floor, which will probably run $75 - 125 per seat as I recall. And most importantly, however you purchase them, make absolutely certain that they are shipped insured. As my experience went, I had to have 2 replaced due to damage.

The come in big boxes, and they are easy to damage...