I doubt there's much difference between an i3 PC and an i3 Mac. I stand by my opinion, but that's exactly what it is...an opinion.
If I were buying or recommending a Mac, I would still recommend the i5 or i7. Pro Tools runs great on either platform, assuming compatible parts. There is only 1 company building Mac's, while there are hundreds of possible PC iterations between the several companies(like Dell, Gateway, Lenovo, HP, etc) and all the custom built stuff. There's just no way to test and approve every combination of PC hardware, while every different Mac could likely be tested in a single day
BTW, if you get an external 7200 rpm drive that has both firewire and USB connections and an Oxford chipset(like Glyph and OWC have) then the drive would be usable on either a Mac or a PC(you might need to reformat if you switch platforms).