Which would you pick between the two DAC.

We are gonna need a little more info Eddy. System? Music preferences? Personal sonic preferences? Current system strengths and weaknesses? etc. 

Dave

Thank you, Dave.  Music will be from PC.  Sonus faber Venere 2.5 speaker, Cary Audio SLI-80 Integrated amp, and Schitt Modi2 Uber DAC.  Mostly listen to vocals.  Don't know strengths and weaknesses.  
One other question, would it be a huge improvement in sound quality from Modi2 to Wadia 321 or Codex?

Good DACs rarely (but sometimes, ie tube vs ss) have "huge differences" in sound character IME but have certain "tendencies": resolution, air and open-ness, speed, harmonic texture, accuracy of timbre, PRAT, soundstage, depth, warmth, bass heft. etc. They all are a unique trade off of the above, tending toward some attributes more than others. Select your key sonic priorities and then advice can be helpful. Auditioning for yourself is always better than advice, preferably in your system and room.

A lot of Schiit and Codex owners here. Perhaps they will contribute opinions on your last question. Wadia used to be a huge player in digital, but changes of ownership and design priorities seems to have diminished their presence (but not necessarily their sound quality?) in the past few years.

Dave

resolution, air and open-ness, speed, harmonic texture, accuracy of timbre, PRAT, soundstage, depth, warmth, bass heft.

= Schiit Yggdrasil

Absolute Sound, absolutely loved it.
http://www.theabsolutesound.com/articles/schiit-audio-yggdrasil-dac/

Cheers George

Every Wadia product I've ever heard sounded wonderful, but so what?  Over the years Wadia has on multiple occasions been bought and sold, reorganized, changed location, etc., whereas, Ayre has been very stable.  Draw your own conclusion.