I have been using my UNO32 for a couple of months and I agree, if you need more power, it beats the Arduino hands down. Diligent did a very good job of setting up the IDE to look and feel just like the Arduino and I found it very useful in running easy projects where speed is needed.
Given the low cost, I doubt I would bother using the old Arduino on new projects. For the same price you get so much more.
Just my opinion
ChipKIT Uno32 is based on PIC32MX300 family
and ChipKITMax32 on PIC32MX700 family:
PIC32MX3xx General Purpose (UART, SPI, I2C, ADC, PMP, comparators),
PIC32MX4xx adds USB,
PIC32MX5xx USB and CAN,
PIC32MX6xx USB and Ethernet,
PIC32MX7xx USB, Ethernet and CAN,
I think PIC32 is intended for wide range of applications like ARM Cortex.