For someone just starting out it probably doesn't matter what car you have since just keeping it on all four wheels will be the biggest challenge. Every time you crash you are going to loose at least 5-10 seconds. So if you crash a couple times a lap it doesn't really matter how fast you are going the rest of the time. Once you can put together full races with only a couple crashes the car will come into play.

The one thing I will say though is the new buggies are nicer to work on and would probably be easier to find parts for at the track if you need them. I can't imagine there being a ton of B5m's at the track anymore but there will be lots of B6d/B6.1d