Don't know much about the OFNA or Sportwerks buggies, the Kanai buggies have been around since the early-mid ninties??? (correct me if im wrong) They are the 7.5 Kanai, Kanai II, and the Kanai III, all versions based on the Kyosho Inferno 7.5. They all have a bad on-power push and some handling limitations, which the new Kyosho 777 series addresses.
I think Sportwerks cars have been out since 2003, they are fairly nice cars and handle great, plus they are a great value for the price. I don't like the pivot ball cars too much, they just don't have much steering but the Mayhem has more than some.
The XRAY cars are great, they have been out for a few years. the XRAY is the easiest build, has the best manual and manufacturer support, and is a very neutral handling buggy. There are two versions: The XB8 and the XB8R. The "R", is a cheaper, cost conscious version that is basically the same, but includes cheaper parts like composite braces, cheap thinner shock towers, a chassis with less machine work, softer suspension arms, different gear ratios, and a few other differences. It is a nice car for the money, but I would rather spend 100 extra dollars to get the good kit than getting the cheaper kit and spending more money in the long run buying hopups for it. The XRAY buggy has great steering (only buggy with better steering is the 777), a VERY plush suspension and is a tough buggy.
Hope this helps!