I said recently I thought Scarlett Johansson was a class act, more so than most of today's Hollywood stars. Seems the producers of Hitchcock thought so as well; she'll be playing none other than Janet Leigh. In fact, with a cast that includes Helen Mirren, Toni Collete, and Jessica Biel appearing surprisingly well-cast as Vera Miles, the whole production is oozing with classy ladies bringing Hitchcock's infamous obsession with blondes to life.
Many of the motifs and perhaps a few of the plot elements will overlap, but I'm probably going to watch both. HBO's The Girl primarily focuses on Hitchcock and Hedren's unhealthy relationship during The Birds while Hitchcock seems to be focused on the production of Psycho... and I would imagine Hitchcock's unprofessional interest in his female leads in general.