Personally I think the GS430 is a great sports sedan.
As for Acura vs Lexus, in all honesty, there is no competition. Lexus makes some of the highest quality, most refined, most reliable luxury cars there are out there. Lexus continually tops longevity and quality surveys. They have the best customer service in the industry just about.
Acura on the other hand, continuously nips on Lexus' heels. The cars are shoddily built (at least when compared to a Lexus, BMW or Merc) they have low quality interiors with fake wood trim. The TL even has vinyl on the seat facings! Their flagship sedan is FWD with a V6! Acura is a great car, but I dont consider it a tru luxury car like a Lexus is. Most buyers dont either. A lot of the market and consumer testings show that Acura is more commonly cross shopped with Toyotas, and normal Hondas, while Lexus is cross shopped with the big boys, BMW, Mercedes, Audi etc.