I've used both encoders for years and never had a single problem with either brand.
I have also used both with a variety of other products (AKI, SJC, GP-Wiz, mouse hacks, analog joysticks, wheel/pedal sets, etc...) all without any problems.
Both companies stand behind their products as well as any company I have ever dealt with.
The differences have been debated for years, but the only ones I've seen than actually MAKE a difference to me are:
Ipac will do USB.
Ipac shift function allows the shift button to do a secondary function on release. (I have mine set to pause).
KeyWiz is less expensive.
KeyWiz shift function can be wired so that individual buttons perform shifted functions.