I was actually wondering the same thing myself recently and the guys at Lambo in Sydney kindly let me drive an 09 560-4 (e-gear) and 07 640 (manual) last weekend. I've been considering my next car and was thinking of giving the Murci a go to replace my 05 Gallardo.
The 640 (which I drove first) was a brutal experience - the V12 has a very deep, menacing note, far more so than my current car. Performance wise the first few thousand RPM were very similar to the Gallardo, but after 4-5k RPM it's in another league altogether - I'm not ashamed to admit I had a 'holy crap' moment experiencing the acceleration for the first time. It's a wide car and I was much more conscious of the car's size driving through the Sydney CBD, but I'm guessing people get used to this after a while. The scissor doors are pure drama and I love them.
Sadly the interior isn't up to scratch in my opinion. Whilst everything is well finished (carbon, aluminium, leather etc etc), the design is very much mid-90s and the lack of modern conveniences really shows in a car built in 2007. I really have a gripe with the aftermarket-branded stereo unit fitted into the centre console - it just doesn't befit the car.
The 560-4's interior, on the other hand, is a far nicer place to be. Despite being Audi units (or perhaps because of it) everything looks like it was placed there at the factory, and things integrate with each other in a way you'd expect from modern cars (for instance, the navigation system displays trip details and next-turn information in the screen between the tacho and speedo). The button layout is more intuitive, the finish seems better than the 640 and the seatbelts and handbrake are in the 'correct' position. The interior is Audi A6 to the Murci's Pagani Zonda minimalist, deliberately unconventional design.
Performance-wise the 560-4 isn't in the same acceleration league as the 640, but it was still a noticable improvement over my 05 and the 3.something seconds it takes to get 0-100 isn't exactly slow. In terms of handling I find the Gallardos very easy to drive quickly, having driven the 05 and the 550/560s on race tracks & private road circuits, and the AWD system is very confidence inspiring. Obviously given the CBD test drive I didn't get a chance to push the 640 anywhere near its limits but it does feel like it would take more skill / commitment / bravado to drive it as quickly.
The e-gear system on the new Gallardos is a million times better than on the early cars; although I couldn't make an actual comparison with the 640 given the car I drove was a manual, I believe the Murci's e-gear is a generation behind that in the Gallardo. If you're going down the e-gear path you'll appreciate the improved system in the 560 especially if you often drive in stop/start traffic.
Obviously to each their own, but after driving both my wife and I were sold on the Gallardo option rather than the 640. The Gallardo is more than quick enough for my level of driving ability, the performance, e-gear operation and exhaust note are all significant improvements over our 05 and the interior looks both familiar and befitting a car at that price tag. I do have an infatuation with the Murci's scissor doors but that's about the only thing I'd regret not getting, and for me that's not enough to sway me over to the 640.