hi,

Except program memory, I've noticed the wider power supply, it works at both 3 and 5 V

Not only. For my point of view, the most important things different from AT89S8252:
- 12kb of flash memory instead 8kb;
- 2.7V power compatible;
- Enhanced UART (now it detects frame error and has Automatic Address Recognition via registers SADER and SADDR);
- Enhanced SPI macro (now it may be double buffered);
- Enhanced Watchdog (now it got the second mode in which it may be switched on once and then there is no way to disable it until reset);
- Now there are four interrupt priority levels (by register IPH);
- x2 mode implemented. Now you may use twice slow crystal for the same productivity (for example, 6MHz instead 12MHz). At least Atmel says so...
- Internal Power-on Reset has been implemented. I do not know how good it works and suggest to use external power supervisor anyway.

Regards,
Oleg