I have used both and prefer Symetrix although using an AVIA is pretty great if you¡¯re looking to finish a coding job quickly (the exporting of touch panel graphics and control blocks was huge for me).
Overall, I think Symetrix is a better DSP and the modules on the application market work great. If you do decide to go with Symetrix, I think the serial modules work much more reliably than the ip command processor.?
best of luck!