This will be a boring answer, but hopefully helpful.
Neither HC nor butter is inherently more fattening. Or I guess, to be a bit pedantic, the question should be “which is more fattening per X?” Cuz after all, butter is just heavy cream with the water removed.
Per amount of food: butter has a much higher fat content, so it wins here.
Per $: I believe I get more calories per dollar with butter where I live. However, to maximize absorption, it has to be well-incorporated into food due to its high fat content. Without being too graphic… if your diet is high in fat that your body isn’t absorbing, that’ll be clear on the porcelain throne.
Per labor/input/effort: IME, heavy cream wins here, because the fats are well-incorporated into water (better than in butter). The same principle applies though—if it’s not absorbing, it’s not well-enough incorporated into food. I usually have to dilute HC with milk to get half and half.
IIRC, it’s cheaper to make heavy cream by mixing whole milk and butter, but I could be wrong.
Ultimately, what matters most is what you enjoy and what’s comfortable for you to eat.
1 year