What are the differences between the original Hexcells, Pro, and Infinite?
Forums report that the puzzles in the later editions are progressively harder than their predecessors.
Hexcells Plus (Feb 2014) adds 36 puzzles.
Hexcells Infinite (Sept 2014) adds 36 puzzles, a random puzzle generator (each puzzle has a number and can be shared), mid-level saves, cross-platform cloud saves, and the ability to load custom user-made levels.