1pc seals better but is a bigger PITA to change, 2pc doesn't seal as well but is easy to change. If you are looking at a SBC to buy, get the one piece block. Like Dorian said, most of the blocks that have a 1pc RMS are also ready for a roller cam with just a little work. Both my old SBC and my current BBC are 1pc rear mains and neiter one leaked/leaks
Chris