chillen,
funny enough......I was considering going back to GM myself just before I bought my 2005 ES 330 (curious why the ES isn't on your short list as well?) I agree that Buick's in general are pretty reliable vehicles......but your not even close to comparing apples to apples here with one being a 'Flagship' RWD full size car to a 'run of the mill' midsize car (be very careful with that supercharged 3.8L V6!....and if you go with the Regal GS, make sure ALL service records are present and accounted for.....including the intake manifold gasket being replaced....cause it's not a question of 'if'.....it's 'when' it goes....and I would try to get one under 80k KM's if I were you).
Not many vehicles out there are as reliable as the LS.....although Lexus' overall reputation (regardless of nmodel) is far better than Buick's generally speaking.
Be prepared to invest in a good set of winter tires & wheels should you decide on the LS.....whatever you decide on, I can help find you something 'special' if your interested. ;) Send me a PM.
Check out these links below.....you might find them of interest...
97 - 04 Buick Regal review...
http://www.canadiandriver.com/roadtest/97-04regal.htm
97 - 01 ES 300 (just for giggles)
http://www.canadiandriver.com/articles/jc/97-01es300.htm
90-94 LS...
http://consumerguideauto.howstuffworks.com...us-ls-400-6.htm