Well I'm not the guy to ask that because I'm not a fan of iRacing, although I've only just dabbled in it. iRacing has a very narrow target customer the online racer. There's no AI only racing online. iRacing is also very expensive if you want to turn it into a hobby and get into different cars and race on different tracks. You also have to pay an annual fee on top of literally hundreds of dollars if you want all the content. It's worth it if you dig the game and the concept and racing online.
The graphics, features, content and playability will be light years ahead in Project Cars. It's a full game offline and online, with fully adjustable driving aids. It also caters to controller users, something iRacing does not really do. I can't speak about the online racing portion of PCars because it's not hear yet but as with most online racing, you have to put in some effort to find good racers to run with and when you do, it's fantastic.
It's two different games and hard to compare. Your best bet is to take a look at what PCars offers and decide if that's what you're looking for in a game. No game is everything to everybody, but if you enjoy racing I think you'll enjoy this game.