this reply is a bit late......and u may have already made your decision
I was in the same dilemma as you when i was looking to buy my next car..i ultimately ended up going w/ the Lexus IS for several reasons...
Design: I think the IS design is much better, more sporty than the BMW 3 series...
Quality: Japanese cars in general are just better quality than any other automobile manufacturers in my opinion...they last much longer...break down much less. Further, Lexus doesn't use cheap interior.....the wood they haev in the car is genuine wood made from Yamaha piano's....BMW uses fake leather...and the seats are much harder.....lexus has much more cushiony seats....
More for the Money: Being a techie myself...i fell in love w/ all the tech options that the car came w/...i think lexus has the best navigation system because it's touch screen....both my parents have bmw's... (750Li and X5) and i hate the idrive wheel you have to use...such a hassle and it takes FOREVER to input addresses.....lexus has packages for their options...whereas BMW, options are individual....i did the math...if you include all the options on the BMW that the Lexus comes with.....the price difference is ridiculously different...BMW being much more pricey
Maintanence: European cars and maintanence sucks......both my parents have to take their cars in for service way too often.....something's always off....even the 7 series has these problems.....and it's the highest class!!
Ride: BMW and Mercedes are kinda known for the stiff steering.....i test drove the 3 series and the 1 series...and in comparison, the lexus had much smoother handling....this is mostly due to the fact that they're more performance cars w/ bigger engines and higher horse power than the IS
ultimately you should ask yourself...do you want luxury or performance??.....Lexus would be the luxury...and BMW would be the performance.....
Prestige: Honestly, the biggest thing BMW has going for them is the prestige.....that was the only reason why i was even considering the BMW...if that were not a factor i would go for the lexus in a heartbeat