I am not very well aware of all AVR family and PIC
But for last 4 years I am working on 8051 based 89s51 and since last 2 years working on ATmega16 of AVR family
Here are the points I knew (hope this will help you to decide the chip)
Similarity between 89s51 & ATmega16
Both are 40 Pin ICs
Both are 8 bit microcontrollers
Both have 4 ports of 8 pin each (which means total 32 input/output pins)
Differences between 89s51 & ATmega16:
ATmega16 has inbuilt ADC (8 channel) which 89s51 do not have
ATmega16 has inbuilt EEPROM which 89s51 do not have
ATmega16 has RISC architecture (in simple language it has less instructions than 89s51)