The Duo model can only be used on the Duos & SCD-ROM2 attachments. The Pro will work as well on those systems, as well as the original Briefcase setup. So basically, either one will work in your case, but the Pro version costs about double that of the Duo version since its more universal.
In sum, go for the Duo version unless 1) you plan to buy a briefcase CD-ROM setup later, or 2) you can score a super bargain for the Pro version.