Ok since you have no idea what a 1.3 rotary engine is im just going to asume that you dont want to mod eaither cars so i wont go down that road or racing and what not...
if you plan on going just a stock car to drive around
id have to say 240sx the Mazda Rotary engine is an AWSOME engine and as a convertable thats a VERY tuff call because its very rare but the problem is that the Engine is VERY difficult to fix if something breaks down and Parts for the engine are almost impossable to find =(
eaither way you go both cars should easily have over 100,000 miles both cars are over 15 years old so both of them have high chances of breaking down
if you REALLY want a convertable id say FIND a 240sx convertable there not that hard to find i bought mine for 2,500 (with 200,000 miles)
only thing is i put a aftermarket engine (Sr20det)