general speeking...
windbg is a weight-level tools,howbeit softice light-level..
all i say here suited for souce code debug,not disassemble.

1.load times
when use symbols,windbg spilt every file into single .pdb file
but sice pack all file into one .sym file.
so,,when u debug a large project,sice will spend many time to load symbles file(in our project,it's 5 mins.)

2.c/s debug
c/s debug is used in many kernel debug,(via COM1 or 1394)
when remote debugger is enabled,sice is very fast than windbg.
becauze windbg will spend time to detect symbols file and more work,
which can be saw in win2k leaked code dir,\private\windbg64\...

3.expand funcs
we can easy expand windbg funcs by design some plug-in tools,
but iceext is only chocie of u.

so,my advice...
when debugged in small project,sice is first choice.