Celia did fine to my mind (and i liked the touch of having Nigel still do Death's voice for her books as well). between Nigel and Stephen I can't take one over the other overall, but I do like certain characters better. I loved Nigel's Death, nanny, Colon, and detritus. But liked Stephen's Igor, Angua and Carrot (to a lesser extent). Both their Vimes and Vetinari's were too similar to call to my ear (discounting very early Vetinari ofcourse.). and I also love Nigel's meek voice. Like when Carrot is scolding the AM and Klatch soldiers in Jingo, or the kids and nobby talking to Death in Hogfather.
I did listen to one Tony book though. A Hat full of sky. One thing I did like about it is the little bit of production they put into it. how the two Miss Level's melded together. and the echo effects on Tiffany when she trying to speak through the hiver. Those little touches added alot.