I would have to go with HDD but I do like DVD's too! Here is my pros & cons
DVD Pros:
With cIOSCORP Metroid Prime NPC,Metroid Prime 2 Dark Echoes NPC,Metroid Prime Trilogy can be played without alt dol files! and you get to chose the difficulty!
DVD Cons:
It's sometimes slow,videos can be clip at times, discs can be scratched when not taken care of properly but I don't have a problem with that. Burning games over and over and sometimes it can be a hassle to buy blank discs.
HDD Pros:
Incredibly fast loading times,Videos are perfect,More Space than an actual disc so you can put as many games you want in the HDD until it's full & USB Loaders can create Channels for games to load them without an USB Loading Application but you can use a USB Loader Applications!
HDD Cons:
Metroid Prime NPC,Metroid Prime 2 Dark Echoes NPC,Metroid Prime Trilogy you can't chose the difficulty it's always on Veteran Difficulty.
but I favor more of HDD than DVD but I do play Metroid Prime NPC,Metroid Prime 2 Dark Echoes NPC,Metroid Prime Trilogy on disc the only way I play Metroid Games on HDD is that I have to play them on veteran mode!