As far as the course goes, it is broadly downhill in the first 5K, then flat the rest of the way-no hills at all. You run through the city centre about three times. There are some cobbles in the city centre but I didn't find them a problem-probably only for about 2K in total. Whilst crowd support is not like London or Paris of course, it's still pretty good. The only part where there's hardly any support is the park which you run through at about 32-36K if I remember rightly. Plenty of refreshment points (though I can't remember what drink was served)
It's a fantastic city, with the added bonus of plenty of cheap flights to Pisa from UK(easy transfer from Pisa airport).
The only slight negative is that the expo is a bit far outside the city centre. You need to get the bus from outside Santa Maria Novella Church (opposite the railway station). As long as your hotel is reasonably central, you can walk to the start. There is no need to use the shuttle buses even though the start is over the river and up a hill.
KJ- I have done both and given the choice I would do the Double or Quit.
The DoQ is not flat, but as long as you don't go too mad too early, it's not too bad. Whether you wimp out or not depends on your mental strength . If it is a hot afternoon, you might be better off doing the 5 only anyway.
The Spire 10 is pretty tough. The first 4 miles iare almost entirely uphill. Even the downhill bits later in the course can't make up for it.