Hey there

I have a few questions for you guys...
I'm buying my first fixie (yay!). I've decided on an Iro based upon some of the threads I've read (and the good prices). However, I'm having some trouble deciding between the Mark V and the Jamie Roy.
I'll be using this bike to commute to and from school about six miles each way over rolling hills, probably in all weather, although I'm not sure how I feel about riding in snow yet!
Does the aluminum Roy frame run much harsher than the cr-mo mach V? I know aluminum stands up better than steel in rain and harsh weather but does it make that big of a difference if I dry it off (and generally keep it clean) when I get to my destination?
As for stem size, what sort of difference does 10mm make? Is that 10mm vertical or horizontal length?
And finally, for rolling hills and a fixie noob, does anyone have cog size suggestions? From my research on here I was going to go for a 16 or 17 tooth, but if anyone has different thoughts on the matter please feel free to let me know.
I hope I haven't bored anyone to death with the questions, but I am, as my title indicates, a newbie!

Thanks much
Christina