My IS felt small when I first got it but in no time at all I got used to it and it didn't feel uncomfortably tight at all. I even took a road trip with 4 adults and we were all fine.

 

It's depend on what you want the car for. Like most already said, the 3 cars has 3 different purposes. The ES if you want space and comfortable ride. It's also depend on how long you want to keep the car. Let said money is not a real factor.
At 30 and marriage is on the way with no kids in mind for a while, then the RC or the IS. I test drove the RC before getting the IS and I didn't like it as much as my Supra (mainly because there is no manual trans). In the driver seat, both RC and IS is very tight compare to the Supra.
If you want kids and fun then go with the IS, but reality will kick in the moment you put in the 2nd infant car seat.
The ES if you want a relax and enjoyable smooth ride and offer little more kids friendly car seat .

 

Why not just get a GS? It's exactly what you're looking for...sporty and roomy (not as roomy as the ES). The interior is way nicer too.