I had a GS350 loaner for a few hours yesterday, and also test drove a perfect condition '07 LS460. Prior to this, I had test-driven an '06 GS430 and '05 LS430 Sport back to back in '06 and thought the LS was superior in every aspect -- I didn't even feel that the GS was more sporting feeling, frankly (it had a more "sporting" seating position and cabin atmosphere, but the vehicle dynamics didn't feel more sporting than the LS430 with Sport Suspension). Despite that experience, I was completely shocked yesterday at how the GS350 felt more like a Honda Accord competitor instead of a minor step down from the LS430 (comparing to my '05). My wife and I couldn't get over how huge a difference there was in the quality look/feel and the trim level inside the car. And on the road, the GS isn't in the league with the LS either in terms of quiet, comfort, or pretty much anything I could gauge. As I've posted here before, I've had a couple of ES350 loaners in the past 3-4 months, and I would say that the ES and GS are very similar in finish and general feel unless you push them hard enough to make the front drive v. rear drive start showing through. Otherwise, there's not really much difference in the driving experience of the ES and GS just cruising.
The LS460 -- Despite not really caring too much for the generic sedan looks of the 460, my wife and I were/are willing to trade the 430 for a 460 (or anything else) if the 460 proved to be a superior car on the road and we aren't too worried about reliability (not an issue with the Lexus cars, but a concern with MB's and some others). I had driven a 460 back when they came out, compared it to my '03 LS430 (I had an '03 at that time but have an '05 now) with the Teins and 19" wheels/tires, and decided that I actually liked the '03 the way I had it set up about as well as the 460, though they were fairly different-feeling cars.
Yesterday's impression of the LS460 was pretty similar to my original. The 460 feels substantially heavier than the 430. As one might expect, this is a slight positive in cases where you are simply cruising, a slight negative when you are turning or driving in a manner where weight works against vehicle performance. All things considered, I prefer the more nimble feel of the 430, and overall on-the-road feel (this comparison was with the factory wheels/tires on the 430 -- with the aftermarket wheels/tires, the 430 has a slightly bigger advantage still). Also, the 460, despite significantly higher HP rating, does not feel any quicker than the '04+ 430, and in fact, I'm not sure it feels quite as quick and responsive in circumstances where you are maneuvering in traffic but not going wide open throttle (it does feel quicker than the '01-'03 with it's more "sluggish" transmission that is programmed for even more luxury-oriented shift patterns). Either way, the difference is minor.
The 460 seems to have more wind noise than the 430. Tire noise seemed about the same (again, I had my factory wheels/tires on the 430 for this comparison, but my aftermarket wheels and tires are essentially as quiet as the factory ones). I was surprised to find that I (and my wife, especially) slightly prefer the softer, wider seats of the 430. I've never thought the seats (comfort) were a strong point of the 430, and on my original test drive I came away thinking the 460 seats were an advancement, but this time I didn't feel that way -- they seem a little firm, but not really like a super comfortable seat. I didn't test the sound system, but I've done that before and the 460 has no factory equal, IME.
All things considered, I think I would take the 430 for my use even if they were the same price.
Edit: ^ After re-reading this, the above is a strong statement and I suppose I'd have to give further careful consideration, look at aftermarket wheel/tire options, and maybe spend a few more miles behind the wheel of the 460 to say for certain. But bottom line is, it'd be a tough choice at the same $. For $20K difference, though.... No.
The only other luxury vehicle that keeps calling my name is the S550....