Next time you're asked a question which starts from "What is better", ask "what is your definition of 'better'?"
There are many considerations when choosing an MCU, and two things which are of the least concerts are "number of bits" and nominal MHz speed. These are marketing things. However, since hiring is done by technically-illiterate HR people, you have little chance to be hired unless you say these things are very important. In real life, the most important thing is periphery - in embedded world you can achieve better performance doing things in parallel, rather than pipelining sequential software threads.
As to the speed, consider these examples:
16-bit dsPIC33 running at 70MIPS doing FIR will leave most of the super-fast 32-bit processors in the dust because FIR is exactly the task it was made for,
Little 8-bit PICs have predictable command timing and can be used for fast big-banging. Since most fast 32-bit processors use caches and pipelines, you can no longer rely on command timing and must use a timer instead, and hence you will require a processor with 5-10 times faster nominal MHz speed to achieve the same result.