I loved the adventure games both comanies released, but Sierra is my personal favorite. For me, Police Quest 1 was what brought me into the world of adventure gaming, and PC gaming in general. Police Quest 1, 3-D Helicopter Simulator, Thexder, and trying to sneak a copy of LSL1 from my dad’s office without getting caught are what reeled me in as a Sierra fan, and it just stuck with me ever since. Like others said, Sierra also covered a broader range of storylines… from all of the major series games to everything in between (Codename: Iceman, Conquests, etc.) Needless to say, I own more Sierra games than LucasArts. Plus, it’s the classic Sierra (Ken Williams era and prior) games that I am passionate about to the extent that I still enjoy reading about them on these sites, even though a game hasn’t been released in almost a decade.
LucasArts released some great games… (the Indiana Jones series (Crusade and Fate of Atlantis) and the Monkey Island series were excellent), but my personal favorite will always be Sierra.