HD, due to the 360.
First i switched to my CRT Monitor, and even though it was technically smaller, i knew i couldn't go back.
I switched to a larger LCD Monitor until i finally bought a HDTV some years ago.
Since then, i had to play Rockband on an SDTV twice and it. is. HORRIBLE.
EDIT: Funny, just two hours ago i heard a work colleague of mine complaining that Skyrim on an SDTV is unplayable due to text size. Gonna borrow her my old 360=>VGA cable, probably with a cryptic message about how "there will be no going back"