your decision on the IS300 is already a wise choice :D
I like the EVO7 myself but the build quality won't match that of the superior Jap . It's 2 different cars. One is sports, the other is comfort and "sporty" sedan.
Final choice is up to you and whcih is more importnat to you. If you are thinking of gaining more power, the Evo is the cheaper option. If you intend to keep the car stock, the IS300 for sure.