The consumable digging implements could vary in the following attributes:
- Power (how many swings to break a block)
- Durability
- Effect size
The size is evident, every pick I've found has a 3×3 size. But I'm not immediately certain how the picks vary in power & durability.
Best Answer
The official wiki, Starbounder, currently answers this question on the Tools page.
For convenience, the relevant text is quoted below (2017-10-05, current Starbound version 1.3.2)