According to [the Book][1]

![Understanding MySQL Internals][2]

Page 198 says the following:

- Paragraph 6 : A MyISAM B-tree consists of leaf and nonleaf nodes, or pages. 
- Paragraph 7 : Both leaf and nonleaf nodes key values and pointers to the record positions in the datafile. Nonleaf nodes additionally contains pointers to child nodes. 

Given this description

- a unique index would have more nonleaf nodes. This would lend itself mode towards ordering and searching.
- nonunique index would requires less nonleaf nodes. This would lend itself mode towards doing range scans (tables and index)
- [A covering index][3] (which would contain all needed columns for specific SELECT queries) would combine the best and worst of both. This would allow for a range scans that would need more nonleaf nodes  and provide for ordering/searching. The additional benefit would be bypassing the need for reading table data if all needed columns are present and accounted for in the index.
- I mentioned other aspect in my past post http://dba.stackexchange.com/questions/20047/benefits-of-btree-in-mysql/20114#20114


  [1]: http://www.amazon.com/Understanding-MySQL-Internals-Sasha-Pachev/dp/0596009577
  [2]: https://i.sstatic.net/BNnv4.jpg
  [3]: http://peter-zaitsev.livejournal.com/6949.html