[Groucho2004]

I found that in many cases 64 bit uses slightly less memory.

For example:
Script:

colorbars(width = 1920, height = 1080, pixel_type = "yv12").killaudio().assumefps(50, 1).trim(0, 499)

RemoveNoise()

function RemoveNoise(clip video, int "threshold")
{
  last = video
  threshold = default(threshold, 300)

  sc = MSuper(hpad = 16, vpad = 16)
  backward_vector = MAnalyse(sc, isb =  true, delta = 1, blksize = 16, overlap = 4, truemotion = false, sadx264 = 4)
  forward_vector =  MAnalyse(sc, isb = false, delta = 1, blksize = 16, overlap = 4, truemotion = false, sadx264 = 4)
  MDegrain1(sc, backward_vector, forward_vector, thSAD = threshold)

  return last
}

32 bit:

Frames processed:               500 (0 - 499)
FPS (min | max | average):      17.38 | 72.00 | 30.01
Memory usage (phys | virt):     102 | 98 MiB
Thread count:                   9
CPU usage (average):            25%

64 bit

Frames processed:               500 (0 - 499)
FPS (min | max | average):      17.76 | 77.57 | 31.38
Memory usage (phys | virt):     95 | 91 MiB
Thread count:                   9
CPU usage (average):            25%