This is close. The Lexus has 300 horsepower, the Acura 270. Both are very reliable. The Acura has a much nicer interior, but the Lexus is RWD (Acura FWD). If you live in an area that gets snow, this would be a pro as opposed to a con. Being newer, the TL is also quite a bit more modern looking.
I'd go with the TL, but the GS400 is a great car too.