can any one let me know which one is tough either 8051 or pic.
can any one make out the difference among them.

The PIC was designed by a bunch of guys that wanted cheap stuff. Then demands grew and there was no natural progression, so they did something different. Then demands grew and there was no natural progression, so they did something different. Then demands grew and there was no natural progression, so they did something different. ....

The '51 was designaed by a bunch of guys that wanted good stuff. For the '51 there is a document ("the bible") the base of which was originally written in about 1990 and like The Bible, it still holds true. All improvements to the '51 (and there are 1000s) have been done as additions, not changes.

Thus, if you stick a 1990 '51 program in a derivative just introduced, it will (after 5 minutes of adaptation) run. DO NOTE, that doing that would be totally silly, the peripherals have progressed, 1990 would require external memory, 2006 does not, 1990 would require a "programmer" 2006 does not, but the thing is that whatever experience you have or gain is always valid.

Now, due to advances in chipmaking the processor "designaed by guys that wanted good stuff" cost about the same as the processor "designed by a guys that wanted cheap stuff" so the choice is yours.

Erik