You're going to get every answer under the sun on this question! I think you should just buy what you like. Both are beautiful cars with different characteristics. The BMW's are more sporty (appearance and drivability). The Lexus is smoother riding and tuned a little more towards "comfort".
Lexus has an excellent reliability history. BMW's used to be reliable, but they're having the same problem Mercedes is currently having....they've gone so high-tech in their computerization, that glitches with the electronics have brought down their ratings.
On the up side, however, BMW maintains its value better than any other vehicle on the road (they surpassed Mercedes in 2003), so lease rates on new BMW's are quite attractive, and they include normal maintenance. I've been quite pleased with mine in that regard. Even the brake pads are covered. Insurance companies charge extra for BMW, though (all BMW's carry a "high performance" surcharge with insurance companies).
In the end, however, I think I'd just take whichever one suits your personality and driving habits best.