Zombie island is a blast but moxxis underdome riot is not that good.
Story: Zombie island is about this crazy dr ned who has made everyone in is town a zombie and it is your job to stop him. Now mad moxxis on the other hand doesnt really have a story other than you have to kill a lot of people.
Gameplay: They are both expansions of borderlands so gameplay hasent really changed much but on Zombie island things get a little creeper and combat can be more frantic than the main game but still the same and you get at least a dozen or so quests to do and is more borderlands just like I wanted but Mad moxxis takes away the fun singleplayer experiance I was having a makes a totally mutiplayer expansion were you go from area to area and kill lots of people and there are no saves so if you want to quit a area you must start from the start again which gets really annoying after the 2nd or 3rd time plus if you do singleplayer like me and you die you must restart to the begining round which also is very annoying so I went and played The secret armory instead since this one is a total waste of my time.
Sound and music: The voice acting in the game is just as good as the main game and is a lot funnier too. Plus the music in Zombie island is creepy and gives the expansion a unqiue feel. Mad Moxxis is a bad game but the music is the best music I have ever heard in borderlands and is a techno rock type music that can get you excited.
Graphics: Same as borderlands except Zombie island is a lot darker and creeper than the main game.
Overall: The Zombie island of dr ned is a great game and is totally worth it if you want more borderlands but Mad Moxxis Underdome Riot is best avoided so overall Its best to buy Zombie island by itself and save your self some cash.