I did the xB because I used to own one. Not the most effective choice, but it got the job done. If I were starting over I would try to get by with the cappuccino, since it is one of the most fun in the game once you can afford to race mod it.
Would be a challenge for some of the early races other than the lightweight cup, but I enjoy a challenge.
Having played both ways, don't over mod your cars, running every race by over a lap isn't that fun. A close competition is far more exciting. 10% less hp, than competition is a good starting point, since the AI drives so conservatively.