It depends what you want to play. I used to Detune my 6-string to play all kinds of metal, and then went on to get a 7-string. Though it saved some hassle whenever I wanted to play something in B (or A if you like Korn), I couldn't play anything in C# etc. without having to rewrite riffs. If its for original stuff, get a 7. If its to rock out on, stick with a hard tail 6 string.

Don't try detuning a guitar with a trem system. Trust me :(