The NES used an ricoh 2A03 which was based on a MOS 6502 core.
The 2600 used the MOS 6507 which is basically a 6502 in a slightly different housing.
In the time the 2600 (77) and the FC (83) were released, this was one of the most popular and affordable 8-bit processor for any kind of computing application (consoles, arcade boards, home Comps etc.), which was later replaced by the superior motorola 68000 (i think same developers were behind that), for similar application fields.
So yes, they were very similar, but which counts for almost any game and/or computer systems from that time.
The Genesis contains the same type of cpus as the SMS and Neo Geo, but it is very different from either.
