The primary difference is internal fragmentation versus external fragmentation. The Reorganize fixes external fragmentation when the 8k pages are physically out of sequence. The Rebuild fixes internal fragmentation when there is a lot of empty space available on the 8k pages. Unless you are querying the sys.dm_db_index_physical_stats DMV using the DETAILED mode, you won't see the data regarding how much internal fragmenation exists.

avg_page_space_used_in_percent:

Average percentage of available data storage space used in all pages.

For an index, average applies to the current level of the b-tree in the IN_ROW_DATA allocation unit.

For a heap, the average of all data pages in the IN_ROW_DATA allocation unit.

For LOB_DATA or ROW_OVERFLOW DATA allocation units, the average of all pages in the allocation unit.

NULL when mode = LIMITED.