I had a yellow Murcielago for a while (not an LP640) - to be honest I felt such a cock driving it that I couldn't keep it for more than about a year... I would get out at gas stations feeling deeply embarrassed.
A beautiful looking car and well made too. Fragile (and costly) clutch, all wheel drive keeps it absolutely planted on the road - barnacle like grip in my experience. But a little like driving the Titanic from the pointy end, a huge car and felt heavy until doing about 70mph +. Not as nimble as the FGT which, in turn, does not feel as nimble as a 911 for example.
Neck-snapping acceleration, but, (and it's hard to define) the GT does feel as if it has 'more' somehow. The power of the GT has been described as nuclear, as linear - very smooth delivery.
Ultimately it's probably not come down to 'which is the better car?'. The GT holds its own in the company of the big Lambo, equally, the Lambo is the epitome of exotic... Rarity? Exclusivity? Style? Hmmm!
There will always be a Murcielago available - can that be said of the Ford GT?
You've already made your mind up to keep the FGT, I think it's a wise decision.