I looked at all the same cars before I bought, as well as the Cadillac CTS, the Acura TL & the Infiniti G37. I'll just compare the ones you've asked about though.
To me the best car in this class is the Hyundai Genesis. You get RWD, a V8 if you want it, a bigger car with arguably a better quality interior. Handling is better than the ES but it doesn't ride quite as smoothly. The downside is that its a Hyundai, honestly. You don't get the brand prestige, you don't get the dealer and support experience, and you don't get the resale value you get with the Lexus.
The Buick is also a nice car, but personally I hate the interior. To me its a mess of cheap materials, fake wood, unneccisary angles and bulges trying to look modern...in a Disney World Tomorrowland sort of way. Nothing elegant about that interior vs the interiors on the Genesis or the ES IMHO. The car rides and drives well...similar to the ES. Again...you don't get the brand prestige, dealer experience, or the resale value you get with the Lexus.
At the end of the day, driving a "Lexus" is a symbol of the hard work I've put in and the success that I've gotten back out of life, and I couldn't bring myself to trade my Lexus in on a Hyundai. Is that small of me? Probably but its the truth.
Have you driven them? What do you prefer?