I know that in the 2006 Motor Trend Car of the Year testing, the most intense tests that new cars undergo in the magazine, the editors preferred the IS to the GS. The IS made it to the final round (10 finalists out of the 28 tested) while the GS didn't make the cut.
Not that i mean to bash your choice of the GS, Jaybeaux, but the IS does have manual shifting on the console as well as the wheel, it does have heated/vented seats, it does have intelligent key, it does offer AWD (250 model only though) and it has 306 hp vs 303. I don't know about the rear airbag thing, though. I have heard that the rear room in the IS isn't so hot because the front seats are so large. You're right; it all depends on what you're looking for. I'd think the biggest issue would deciding if you need the space; more than one mags have said that the IS is essentially a baby GS.
Here are those articles for a little light reading.
http://www.motortrend.com/oftheyear/car/11...s_is/index.html
http://www.motortrend.com/oftheyear/car/11...s_gs/index.html This tests the V8 model instead of the sixer, sorry...