Smaller than mATX, cheaper than P4 ITX (processor is included with VIA)
Uses less power
Produces less heat
Small heatsink, quieter fan
More expensive, or larger mobo.
Uses more power (processor and mobo)
noisier fans required
People choose the VIA usually because it is smaller and cheaper. Depends if the VIA will do what you want it to. I switched from VIA to mATX/Athlon because I felt it needed more oomph.