Er, there's not really anything more inherently "agile" about aluminum bikes...what makes a bike agile is the frame geometry. Also, going to aluminum/carbon over steel will save maybe two pounds, not 10. Does your Volpe have wider tires than your friend's bike? More tread?
To the OP: I think you're on the right track thinking about fit, but when you're in between sizes it's probably better to go with the slightly smaller frame and get a longer stem and maybe a seatpost with some setback.