this has been answered tooo many times, please search next time.
both hard mods and softmods are just different ways to allow unsigned code to be run.
there is no difference in performance between a softmodded and hardmodded xbox.
the advantage of a hardmod is ease of recovery and hdd installation, with a hardmod you can mess with the files all you want and you can just boot a recovery disc no matter what.
the advantage of a softmod is that you dont void the warranty.
simple as that its choice. i personally prefer hard mods, but i love to just play around with things and if i screw up i can recover easily, if you just want a modded xbox, xbmc, backups etc, then perhaps you should look at softmodding.