I've driven both.
I'd got for 330i ZHP.

The IS300, to me, feels more like they rebaged a toyota than anything else. ( I understand that Lexus is Toyota, but more so that they BUILT a toyota in terms of refinement and feel and slapped on the lexus logo.)
I didn't like anything about it other than looks. Or if you were going to go big turbo, then duh.

I have a <3 for ZHP package 330's just not fond of the all black interior, looks almost cheap to me, I'm not sure what you're spending, but most ZHP packaged I've seen are low-mid 20's? Unless you are going high mileage 70k+ in which case I'd still fork up a few more bucks and buy a M5 or 02ish M3.

It was one of the most amazing cars I have ever driven (M5 that is)