Selecting Time period with multiplexer
Some ideas (I liked idea 2 and 3 better, since they are easier to do digitally):


You can have a local oscillator with something near a 250-500MHz signal and a PLL. Then you put a comparator in the phase output. Calibrate the comparator to the exact frequency difference you want. The comparator output will be the mux input (maybe inverted, depending on mux type, but you can always invert the comparator inputs).
Use a Monostable multivibrator. Trigger it with the input pulse. Tune it to return at something like 1.5ns. When it returns, if the input is still up, it is slower than 1.5ns. If it is already down, it is faster. Use the vibrator return as trigger to a logic comparator, making the output of this logic the input to the mux.
Use a faster-than-input counter. When the input pulse starts, start the counter. When the pulse finishes, stop the counter and use its result in a comparator. If it counted more than n times (counter frequency-dependant), then you should switch the mux. Use either the A&gt;B or A&lt;B outputs from the comparator as the mux input. Don't forget to latch the counter before the comparator (use input as triggers) to avoid hazards while counting.