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.
@vahnj Yeah, that's a fair point! (Also, huge bonus point to you, I totally forgot Codename: Iceman even existed) Accessible is probably not the right word, I think Sierra's bad about scattering stuff all over and backtracking and gotchas, but LucasArts is more prone to very poorly telegraphed adventure-game-logic as far as getting past particular points. Or maybe I'm just really annoyed at Grim Fandango right now. ;)
@vahnj Elaborating on this, I don't think "You can't die" actually has any effect at all on difficulty; for all it removes specific gotchas and forced reloads, it does at least allow you to narrow the solution-space sometimes. Banging your head against the same three screens for hours with zero feedback is intensely annoying in its own way.
@indi For a game that has death handled responsibly, see "Dragonsphere", which checkpoints deaths for you.
@vahnj Oh dang yeah, good idea. And I hadn't even heard of this one, I love running across obscure ones. Did you ever play the Gateway ones, or the Callahan's Crosstime Saloon one?
@indi I don't think so cuz I don't remember, and nope!
@vahnj Those hit double-nostalgia for me since they're based on books I read growing up, the same time I was playing a lot of adventure games. XD
@vahnj Thanks for getting me started on this by the way, and sorry if my responses have been a bit overwhelming! Ever since I played Hiveswap (which I found actually really good, (if short) if you haven't tried it), I've been thinking about them a lot and playing through some I'd missed, and watching LPs and stuff. :)
@vahnj Er. CAUSES specific gotchas/reloads.
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 honestly- as a person who prefers Sierra- Sierra games have a lot more bad puzzle designs. Gabriel Knight 3 and Codename: Iceman are two of the worst offenders I can think of off the top of my head
I don't know if I'd call Sierra more accessible, but yea, Lucasarts does tend towards a little more zany puzzle solutions