There is more to processor speed than MHz. MHz simply means how fast it can process one command, or rather how many commands it can process in 1 second.
1MHz = 1000000 commands per second.
Then there is the internal memory. The bigger the internal memory, the less time is wasted accessing the RAM. So it kinda depends on what its actually doing to be determined faster or not.
If you have 3MB of commands with a 3 MB internal chache and no branching. The more MHz capable CPU wins.
If you have a vastly bigger set of commands, then at some point the faster CPU with less internal memory will waste so much time going though the RAM and back, that the newer CPU with more internal memory will outperform it, because it needed to access the RAM a lot less.