Lots of discussion has taken wiz a wiz 8051 & PIC

Both at adv & disadv which vary according to application needs . But two biggest distinctive adv/disadv that is making soem to stick to 8051 & other to look for PIC solutions are

(1) 8051 - It main architect is very well defined and doesn't change with derivative . Whenever a new featue is added (generally in from of Spec purpose perip like PWM, SPI , CAN) you just have to get relevant address associated registers to use that peripheral or feature.
You don't have to go through cumbersome learning process again & again
You can coutine to use old assemble & compiler but just have to define relavant address in file
You can continue to new variant for old applica/design(without much effort) (if new one cheaper & easily availa) as well as new one thus maintaining inven of new one

PIC - They have go so many series 12 , 14 , 16 ,18 all deffering with one other.
Whenever PIC want to introduce new feature or come out with moe power full varia they will introduce a new series which may be quite diffe in previous one and then one has to invest in learning & tools , re-engg old software etc
(Same is the case with ST they have got ST5, ST7, ST10 etc)

(2) 8051 - But 8051 vendors are slow in coming out with new exciting features as per current requirement / trends /design need ( that too in low cost version)
example almost all verndor(PIC- 18f4331, Motorola - 65c908, ST- ST7 MCU) all come out with full fledged dedicated Motor Control Unit(MCU) peri which generate 6 phases of PWM waveform for 3 phase VF AC drive application no 8051 vendor has come out with dedi MCU peri though demand for motor control appli is growing very fast.
Most other vendors are coming out with complete refernece solution/ kits( which can be deployed to real world solu without much change)for appli for which microcon is intended for same is lacking is case of 8051 vendors.

Hence architech & intruc set stability/compatability making 8051 user to stick with it
But those can effort for constant change over can certainly benefit from versatlity provided with PIC , ST , Motorola micros variatns

Anil