The maximum permissible towload for a 1.8 litre Astra Cabriolet is 1300kg. However, I'm a bit wary about towing with any convertible, whatever the make or model. They are not as torsionally rigid as a saloon car and the added load of towing can put very high demands on their structural integrity, especially on bad roads. I'd play it safe and tow with the Hyundai if I were you.