I test drove both the 320i and is250 before deciding to go with the latter.
I can tell you this much, the 320i is crap. It lacks ALOT of power and smoothness and you will not be happy with it. Under normal driving when i drove the 320i, i can't help but go over 4000rpm (just to get it to a speed i want)...this is bad. The Is250 had no problems and is alot more responsive.
If you want to compare a bmw to the is250, you need to get the 325i, and that would put you near 80K. Not worth it for just an engine upsize with everything else the same.
BMW is very boring to drive and lacks alot of features.
The Is250 is for cruising in comfort and fully loaded with options your heart will ever desire so if you're after a car that would give you best value for your bucks, go for the Lexus.