A smaller pipe = more head (pressure) loss = less pressure to move the water = less heat. Go with the 3/4 pipe. If you're using pex be sure to get the pex with the oxygen barrier, slightly more expensive but absolutely worth it. Just btw, I'm wondering if you're system is hot water or steam and why you decided to switch out the copper baseboards for iron radiators?
I'm using Slant/Fin Multipak 80, 730BTUh / linear foot @ 180 degree F.