I recently bought a 2023 ES 350. Before buying, I also test drove many different cars, and the IS 350 was one of them (there was no IS 500 on the lot at the time). Both test drives were taken long enough to judge them on some curvy roads as well as a short highway jaunt, then back to the dealership.
Obviously, two very different missions between the ES and IS. I chose the ES because of the room in the cabin as well as the trunk; that's what I needed. It is an extremely competent and supple car which envelops you in a cocoon. To get more room and comfort, you'd have to go WAY up-market. The ES is a relative bargain in terms of ride/size/comfort versus what you'd have to buy in another brand if size and comfort are your primary goals. (Nothing's cheap any more, so "bargain" is a relative term here).
What I "wanted" was the IS 350 F-sport. Really was impressive on the test drive; can't say anything bad whatsoever about how it drives and feels. But if you need trunk or backseat room for adults, look for something else; the back seat is very cramped and the trunk is not suited for long trips with several people. The IS certainly rides well for an older design; taught yet supple. The IS is a very dated design; it goes back about a decade or so with no major revisions other than some minor aesthetic changes and tech screen updates.
- On the plus side, that old chassis is well sorted out; any problems have been long ago addressed.
- On the down side, that chassis is very old, and doesn't have the ability to compete in terms of handling dynamics with the other players in its market class (small RWD sedans from Germany, and the current TLX).
That doesn't mean the IS isn't worthy of consideration. Just realize that the IS won't be the pinnacle of available choices against other brands.
One thing that also pushed my decision to the ES were the F-sport seats; they are tight for a bigger person. I'm not tall, but big in the hips and chest, and I found the F-sport seats to be too confining. If you get the "regular" IS, the seats are more accommodating, but then you're losing all the appeal of the IS chassis (sport sedan handling) that comes with the F-sport performance package. They also offer an F-sport appearance package; whey someone would pay for the looks and not the "go-go" is beyond me ....
Then there's the IS 500. I've not driven one; probably a good thing or I'd then own one, and get into all manner of shenanigans. So someone else will have to comment if they've had experiences with the IS 500, unique to its drivetrain. Plenty of YT videos for that into.
Because I already have a true sports-car (MX-5 ND-2), I didn't need the IS "sport" sedan as much as I just needed a good "sedan", for daily driven stuff and long trips (IN to AZ and back). The ES made more sense for my application. If you want some sport with your sedan, the IS is a very nice vehicle, but just be aware of the compromises it brings to the table.