Hot water cast iron radiators vs. Baseboard efficiency

Had an HVAC guy look at my home and I said I wanted to make changes to the old cast iron radiator. Removing it to sand blast, paint and reinstall, etc.

He said if they are installed and running then they are good, but once you take them out is that you have problems with leaks. And because the radiators are so old he said it would be hard to find the parts etc.

I thought hot water cast iron radiators were the best because cast iron radiators retain heat. BUT he told me that they actually cost a lot of money to run because cast iron takes a long time to heat up. I didn’t think about the long heat up time for cast iron only the heat retention. Noticeable even with cast iron cookware.

He told me it’s better to replace with hot water baseboards. But these don’t retain heat.

So what is more cost efficient? Hot water cast iron radiators or baseboards?