Is there a difference between this and the mod? $30 is pretty steep for an alpha game built on top of one of the clunkiest game engines out there.
The majority of the work has gone into improving stuff under the hood. It definitely feels less clunky, but there's still work to be done. Supposedly, they are getting the guy who did the stance system in ArmA III to help out with DayZ. In terms of FPS, a lot of things have improved. The whole server-client relationship has been overhauled, so that your framerate is no longer dependent on the server. This has also greatly reduced the amount of possible hacks. The game has been improved a lot graphically, and there's still a lot of room for optimization.
The entire map has been revised, so that now every building can be entered and looted. The map has been expanded, and new towns have been added. Loot has been redone completely, so that it has a lot more spawn locations in each house.
The inventory system has been reworked completely, it is basically much much better. There is now a fleshed out clothing system, where different articles of clothing provide more or less inventory slots, as well protection. A lot of work has also gone into the hunger/thirst/blood/health system. A lot of factors play into your characters health now, and survival is definitely more complex than just chucking soda and eating whatever animal you come across.
Overall, the game has less features though. There's only 4 guns, zombies and loot do not respawn, and some houses do not even spawn loot to begin with. Some towns in the north are still being worked on. Right now, the fun of the game is the player interaction.