Just a detail here, but the slowest STM32F4 (wich is a 401 if I'm not missing one) can run at up to 84MHz, and the fastest ones are rated to 180MHz, so 64MHz is not quite "full speed" (or did I miss a model?)
Other than that, as already said. If you're not using low-power modes and run at full-speed all the time, a slower clock speed will invariably result in lower power draw.
If you're using low-power modes, then it can depend on the "duty cycle", overhead of waking-up, etc.
Unless you have specific reasons to clock the device at a higher frequency than needed, I would suggest lowering it. Just provide some headroom as needed depending on your timing constraints. I don't like wasting energy for no reason.
Look up the "Supply current characteristics" section in the datasheet to get some figures.