First thing I'd do is look for a local specialist and have them do a pre purchase inspection. Either car would be a good choice but they need regular TLC so you want to steer clear of the abused ones.
I'm not too familiar with early LS400's but I know a little more about the Q45's. In Atlanta we have a superb indy specialist that only does Lexus and Infiniti, T3 Automotive. In talking to them I know they feel the LS is the more reliable of the two but the Q is more fun to own and drive. Neither is cheap to repair but the Lexus prolly has the edge here.
If you want more inside info on the Q45, check out
www.nicoclub.com and read posts from "Q45tech" or find posts from "Q45man" at
www.edmunds.com . This guy is Dennis Evans and he is the Uncontested Supreme Knower Of All Things Q45. Just search his posts and you'll get a graduate course in the 1st gen. Q45.