LaStarza was zero dimensional. A protected powderpuff Fancy Dan whose entire career was based on drawing with a neanderthal amateur midget. Mac Foster was a man-sized man who had major pop. Good enough by far to beat the Batman bit player.
Foster had dangerous power and a sturdy chin. Lastarza had good defense and stamina. Mac had the advantage in size. Neither can be credited as beating strong opposition however. I’m guessing this is a coin flip.
LaStarza was one of those who peaked early then fell off, like Rex Layne, they both eventually lost to guys they should never have been losing to because they never recovered from their Marciano knockout losses, and they get judged for the losses after Marciano because they were still young, but it’s important to look at the context which was that they never recovered the Marciano losses mentally.