James Corbin wrote:
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"Anybody here ever worked with a 6502?"

Yes, but it's been about 25 years, so take my comments with a grain of salt.

"How does execution speed compare?"

I would put a 1MHz 6502 and a conventional 12-cycle 12MHz 8051 in roughly the same class speed-wise.

"Second, for example if I wrote a routine to multiply a 64 bit number by another 64 bit number on both controllers."

Nitpicking just a bit, but to be clear, the 6502 is a microprocessor and the 8051 is a microcontroller.

"Would there be a LARGE difference between the two (size and speed wise)?"

The 6502 system is undoubtedly using external RAM and the 6502 addressing modes to access that RAM are more flexible than the 8051's. If you can keep the 8051 math routines using internal RAM for temporaries the difference may not be too great. One 8051 advantage you may be able to exploit is its MUL instruction for which the 6502 has no equivalent, IIRC.

In the end, I would recommend getting yourself some headroom if you can afford it, and go with a >=24MHz 8051 and/or one that has cycle-reduced instructions.