Apogee Duet versus a/d converter for Focusrite ISA220

Hi First time user here: tried for a few days to get info..no luck yet. Seems I’ve finally figured out how to post? Checked other threads, pardon me, here goes. I do VO’s, have a Neuman U87 and just bought a Focusrite ISA 220. Am I better off with an Apogee Duet or the optional a/d converter sold by focusrite for the 220? Leaning towards Duet, like the flexibility for travel, etc. Using a Macbook with one firewire. Also, is it true I can’t use an external hard drive along with apogee as Macbook (one firewire) won’t accept it very well? Thanks so much.