I just sold my 2002 GS430 and bought slighly older LS400.
MPG is slightly better on LS, but fuel costs are irrelevant in this segment.
Both cars are very very quick 0-60 and beyond.. i don't think there's much difference between the two.
Personally i find LS more relaxing to drive, but i miss roaring engine noise on GS.
Reliability wise both good. On GS make sure ALL the electrics work and go for the model with satnav. Test drive GS on rough road at slow speeds and also open it up on motorway. Check for suspension noise, seized calipers, wheel whobble, wheel corrosion and stone chips front end. Service history most important.