You can use XMega parts with big success. PIC controllers divide the clock on 4 to execute instructions (so 20 MHz clock results in 5 MIPS performance) where AVR parts use directly the clock - 20 MHZ results in 20 MIPS. So even smaller AVR controllers like ATMEGA88 which run on 20 MHz can outperform the PIC18F252. AVR timers are more complex than PIC ones, you have plenty of RAM - I think you'll not sorry about moving to AVR.
If this is your only specification, then almost any microcontroller has built in UART.
But at least you should additionally specify expected baud rate.