I'd offer the 3.5 gallon is great for the following:
- Brushes/Wheel Woolie. Fill up the bucket enough to cover the bristles, and they can sit in there between tires/wheels, making it an easy way to carry them and keep grime from your other wash/rinse buckets.
- Presoaking towels before a wash. Since the grit guard fits, too, it'll help hold them down in the water.
Bottom line, as Chris and Robert mentioned above, it depends on what you're after. Pretty sure you'll find a use for the smaller, 3.5g buckets in your arsenal.