Like every other Cincinnati situation, this one has never been truly settled.
Last season, Chinedum Ndukwe was used in nickel and dime packages as a roving safety/linebacker hybrid for most of the season. Marvin White saw much less time when both were reserves. Late in the year, when both Dexter Jackson and Madieu Williams were out with injury, Ndukwe played SS and White FS. Ndukwe was the more dynamic player (11 solos, a FF, FR, 2 INTs and 3 PDs vs White's 8 solos and 4 PDs). That fueled speculation that Ndukwe would win the starting job and be more likely to be used in the box than White.
In the meantime, Ndukwe was injured early in camp and White established himself as a solid free safety making plays in coverage and run support very much as Madieu Williams did before him. And the Bengals have since voted Dexter Jackson captain. That's not to say that Ndukwe can't force himself into the picture at some point in 2008. But we still don't know what his injury is and we're continually told that he's "close" to practicing, a refrain that we've fallen for in the past with guys in Cincinnati.
Right now, Marvin White holds all the value that Ndukwe could have had if he had forced himself into the picture. He's the unquestioned starter at FS alongside a so-so tackling SS and suspect LB group. Ndukwe may still be rightly considered to have the higher upside, but he's got multiple obstacles in his path. My bias in dynasty leagues is to consider a one year window in most cases. So, for me, even though I remain hopeful for Ndukwe's prospects, White is currently the much better roster option. Be ready to move on Ndukwe, though, if things start breaking more positively for him.