My bride is 5'3" and says she prefers the box with back rest.
Reason is because it is higher, wider, and she 'feels' it is sturdier.
With the backrest only, she complained about how it hit her lower back when I hit bumps/rr tracks. She still doesn't like riding, but of these two choices vs no backrest, the box wins.
You can also get the wrap around back rest which are softer and some come with arm rests.