I'd prefer diesel over petrol any day. Even with the free market, the price hike for diesel wont be that high, as diesel is used for transportation and it would lead to inflation. I think this system wont affect diesel.
People talk about high purchase and maintenance cost of diesel. But i think diesel fairly compensates that with low cost of diesel and better resale value at the end. But then it depends on one's usage. For ours, buying a petrol would be a disaster.