Fred heat wants to go where it's cold and it every material has a different rate that heat will move through it. The thicker it is the longer it takes to go through it. You're not going to gain anything with efficiency from the thicker steel as far as "holding heat", it's just storing it a little longer before releasing it into the room. It may be a more pleasant environment though without large swings. For example we have steel radiators in this house, the fins might be cast I'm not sure:
My brother put in these really small "high efficiency" baseboard tubing. There might be some gains on efficiency but only because you may not be heating areas like where the return line is since it has already extracted the heat. My Father wanted to put them in here because he saw how fast they worked and the first words out my mouth is "What are *censored* nuts?"
You're not going to gain or lose anything with either sytem, the heat is in the water and it has to go into the house at some point. With the larger baseboards you can maintain a much more even heat over longer period but they take longer too heat up.