bbbb - sorry for the late reply, I've been away for a few days.
I had a Murcielago for a while...
Ooooh, it's so difficult to advise! The Murcielago is a beautiful car - fabulous, art. However it will date quickly as Lamborghini upgrade the Gallardo and update the Murcielago, I guess you'll soon find you're driving last years model. However, this is not really too important in the short term.
I don't have the specifications on both cars to hand, but I'd say that the Murcielago feels a much heavier car to drive - it's quick (no question) but it's 'urgent' compared to the GT. It's a stressed and twitchy beast - mechanical. Way too wide and personally I felt a bit of a dick driving a bright yellow car with scissor doors! But it's art on wheels.... You can feel the passion and breeding. It feels a lump sometimes - too heavy to have the grace of a 360 (for example) or the 'pointability' of the GT.
The GT is different. I love the GT, like I've not loved another car. I really liked the Lambo - but the GT is the real thing. It's the sublime marriage of design and engineering. It's right.
Pick any angle, and I defy you not to enthuse. It's 1960's but will never date - it's such a classic.
Driving it is wonderful - take a spin in one. The power is linear - like the hand of God pushing you along. Nuclear. Easy gearbox and it feels bullet proof. The whole package inspires confidence.
My advice to you? Enjoy the Lambo, thrash it, turn up to ALL your favourite restaurants in it, ensure EVERY friend sees it, do one long trip in it and quench your thirst for this car. Sell it and get the GT - you'll love it.
And that advice is absolutely sincere.
Oh and PS... I think the GT is better built. Looks it to my eye - the engine bay in particular.