I just listed the basics in a summary. there are a few others, but the Luxury package on an IS is considered rare, as they were special order mostly. And it would say Mark Levinson on the head unit itself if it had the ML option. I've heard both, and the premium sound system sounds great, with the ML offering a few DSP sound modes and 5.1 Dobly Digital.
The ever so popular IS350 vs. G35 debate. Well, I personally think that they came out with the G37 because the G35 was getting it's butt handed to it by the IS350. Engine power wise, they aren't even in the same league. And it's still faster than the new G37's. Not to take away from Infinity though, They are a good car also. I just think the IS is so sexy and class. The G35 looks good, but only as a coupe, where as the IS looks hot as a 4 door.