From personal experience I would go for the B10 V8,the E39 is the best looking
model that BMW have produced.
It is a truly Grand Touring vehicle,I bought mine from Germany last year and is
the best car I have owned.
I tried the M5 before deciding on the Alpina as the Alpina is so much easier to drive it has a better suspension setup than the M5 unless you like a hard ride.
Touring France last year I averaged 25 MPG and at times had her up to 170 mph
on the motorway,this would leave an M5 for dead.
If you want a vehicle which is a hoot to drive very quickly in absolute comfort
which can take 5 people and all their luggage go for the B10.
Since owning my B10 the only expenditure I have had is on fuel.
I have also found it very easy to manoeuve and parking is not a problem.
If you would like to see one in the flesh send me a PM.