Both moths are considered hummingbird moths and both look rather similar. There are often color differences, but they are not always reliable. I have found looking for these three indicators to be very helpful when trying to confirm an ID:

Discal Cell

One of the most reliable indicators is to look at the discal cell (a cell along the top edge of the wings). Of course, this requires a photograph taken at a fast shutter speed, because the wings are usually just a blur to the human eye. The discal cell on the Snowberry Clearwing is empty/clear and undivided. The discal cell on the Hummingbird Clearwing is shaded toward the thorax and split into two cells toward the wing tip.

Legs

Leg color is another reliable indicator of species. The Snowberry Clearwing has solid black legs so dark, they are unmistakeable. The Hummingbird Clearwing has generally paler legs, colored white/yellow to red, with the front legs often lighter than the rear legs and being a better indicator for identification purposes.

Thorax

The Snowberry Clearwing often has a dark black stripe on the thorax extending up toward the eye. The Hummingbird Clearwing generally has a pale colored thorax with little to no striping visible.

There are other types of Clearwing moths in the Hemaris genus, but these are the two that I usually see in my neighborhood. Today's photos of the Hummingbird Clearwing are in the post gallery below.