Hi Pando.
This is what I read once in Metalink:
If you want to move the index to another tablespaces => REBUILD
If you can't afford the space to have 2 copies in a moment => COALESCE
Rebuild is best coz decreases the B-Level but has table locking and space requirements.
Coalesce is second best, it is good to reclaim space only.
If in your query PCT_USED column of index_stats is, say, below 40 then coalesce first, because it is faster for large indexes, reanalyze, if you think it is still necessary to rebuild then do it
Hope it helps