Hi,
I drove both (after ordering the xfire) i did not enjoy the manual xfire, the gearchange was notchy and difficult to shift smoothly and the clutch was very light, so along with mercs reputation for poor manuals i opted for auto and do not regret it. 80 miles a day on the M25 an auto is essential and sadly the same is true for most commutes, once in the open you can use the manual shift option for more fun but even leaving it in auto is quick.
The 350Z was a faster more powerful car no question, but i found it hard work, a drive to work everyday in the 350Z would have been a problem with its higher in cabin noise and less comfortable ride and poor interior finish.
I think if i'd been looking for this purchase as a weekend car and using public transport for my communte the 350Z may have stood a chance but if you intend to use it daily the Xfire is the best choice IMO.