enjoyable

Your stated priority list seems to favor the Lexus but your stated car needs favor the BMW. IMO, if you buy the Lexus, you'll always wonder what you missed by not choosing the BMW. Buy the BMW and an extended warranty, follow the maintenance schedule, and enjoy it for a few years until the warranty expires. After that you can buy reliable (and some BMW's are reliable, they just aren't as ubiquitous).

I've owned German and Japanese but I only remember the joy of driving German.

The IS250 simply doesn't have enough engine for this class of car. The IS350 is much more fun to drive than the IS250, even for people (like me) who aren't all that into horsepower.

For something that combines aspects of each, check out the Infiniti G37.

With any German car 6+ years old, you've got to have a tolerance for 1-2 repairs per year. Some people take these in stride. Others...shouldn't be buying German cars of this age. So it really depends on which sort of car owner your are.

Have you extensively driven the candidates? If so, how much more did you like driving the BMW?