Hi Dave,
Despite the fact I currently drive a BMW, I'm still actually quite a fan of Lexus too - my brother has an IS300h and he loves it, says it's the best car he's ever owned. He gets around mid 40's mpg and the tax is very low (£20 I think?). He has an F Sport version which is probably the nicest looking and has some cool features but he also considered a premier as it has all the gadgets. I think Lexus often do 24 hour test drives so you could try living with one for a bit too.
Of course, the BMW is also a nice car and a great driver's car, though I'm not personally a diesel fan I'd probably be looking for a 6 cylinder petrol version.