My son has just passed just ARKS test so we're going through the same decision process.
Most of the advice we've had is that Rotax is more expensive to buy and a lot more difficult to set up for a beginner.
If you want to try before you buy I can recommend
www.srsracing.co.uk who will run you a kart for a test day.
These are at Whilton Mill on a Wednesday and you get about 15mins every hour with plenty of help and advice.
They can also do your ARKS test aswell.
Also have look at
www.kartmania.co.uk at the end of this month which will be a great place for advice and buying equipment etc.
Tony