Pedals for very different purposes. What kind of riding do you want to do?
The R540 is a road pedal: larger base, lighter, designed for riding long distances, a bit harder to get in and out of it, it uses road shoes (3-hole shoes) which are generally stiffer (good for riding) but not as walkable (bad if you want to walk around when you get where you're going).
The M324, as you said, is a "campus" or commuting pedal: it uses mountain shoes (2-hole shoes) which are much more walkable, and also gives you the option of riding the platform side with no cleats at all. It's great for a commuting bike, a bike that you want the option to ride regular shoes, routes where you have to stop frequently, etc.
You can use either pedal for either purpose, but each will excel in their own unique purposes.
Also, they require different kinds of shoes. The M324 uses the SPD cleat, which is a 2-hole cleat. The R540 uses the SPD-SL cleat, which is a 3-hole cleat. There are some shoes which have holes for both types, but either way your shoe needs to have the required holes to match your pedals.
So do you want walkable shoes? Want the option to ride regular shoes? Want the best/lightest solution possible for road riding? Do you have 2-hole or 3-hole shoes? These are the questions you need to be asking yourself.