Lets view it this way...
You got a guitar last B'Day, an amp for Christmas, and another guitar this B'day.
That's still more stuff to add to your guitar collection.
And you got a XBox last B'Day too! So you would be upgrading both of last years gifts.
And, like last Christmas, you would get a guitar accesory (if you get the pedal for Christmas).
So, in my humble opinion, get the 360 for your birthday, and shoot for the pedal for Christmas. That way, you have a larger video game selection (+XBOX LIVE) and a new guitar style!
and
for your B'Day would rock. I hate to say this, but I gotta, best of both worlds, lol. Epic Guitar + Epic System = Epic Birthday
EDIT***
Sweet, my signature works! lol