ah yes, the age old question:
vs 
To actually answer your question (Since she has since convinced me to play some LucasArts games), I'm honestly torn. I think generally Sierra has puzzle design that's more accessible from moment-to-moment, whereas I do feel like LucasArts is responsible for more of the truly headscratching combination puzzles (I'm slowly making my way through Grim Fandango right now actually and DANG there's some abstruse stuff in there)
On the other paw, Sierra is prone to plenty of 'screw you' design things that are pretty infuriating in their own ways; I think unkindly of the MULTIPLE reflex tests in Space Quest 4 for instance, and just the presence of deaths (gotcha and otherwise) in general.
Sierra pioneered a lot early on, which is important and valuable, but also seemed to run out of steam before LucasArts did; their attempts to try new things were a lot less successful than LucasArts' were, and the current revival now seems to be much more thanks to LucasArts folks.
@indi amen