Focusrite pro40 and ISA One Preamp help?

I have a focusrite pro40 interface.
Im looking to get a new preamp for vocals.(Hiphop, Metal vocals, etc)
My Mic will be a SM7B.

I found a deal for a brand new Analog ISA One for 390 out the door.
Now I could get that and run its TRS output into the Line in of my Pro40 and get a decent result from what I understand.

Or I could get the digital combo version of the Isa One(750) with SPDF/ADAT out and run it into my FocusritePro40.
The advantage of that^ would be that I can use the A/D conversion of the ISA one instead of the pro40 and I gain an extra pre amp for all in one practice jams with my drums and guitarist, The jitter clock of the Pro 40 is <250, the ISA One is <20. Downside is the cash haha.

Is that upgrade on the D/A conversion worth it?
Would the sound quality be that huge of an upgrade?
Or should I just get the ISA one Analog for now and upgrade the focusrite Pro40 at a later date?

Im open minded in all aspects, Im also willing to get a different pre if you guys think there is a better option. 400 is what I have now and I hear the ISA ONE compliments the SM7b wonderfully, Im willing to wait longer though if the quality is really that huge of an upgrade.

Thanks for the help everyone!:)