I've heard it said that the best clock speeds are in the range of multiples of 7Mhz. That sounds reasonable as it then matches with the speed of the rest of the system i.e. not having to waste clock cycles.
The A1200's 020 was clocked at 14Mhz - there was one 28Mhz 020 board and there have been several 28Mhz 030 boards.
