I have heard both preamps. I owned the Cary and have heard the Modwright in another system. Tvad's description of the Modwright's sonic signature is accurate. I would say the Cary in stock form is more musical with lush mids, softer bass, and very three dimensional.
Given the right combination of NOS tubes in the Cary (I liked the Raytheon VT-231 in the front position and the Sylvania cryoed 6SN7GTB in the rear position), as well as matching it with the proper amp, the Cary is really a fantastic preamp and great value, especially if you find a nice used one that may also have some upgrades from the factory.
What you're really comparing here are two preamps with not much in common from the perspective of their sonic signatures. My tastes favor the Cary as the better preamp and I'd still own it if I didn't run into a Joule Electra LA-100 MkIII.