By default, WordPress uses the database's default database type. WordPress doesn't pick a type, in other words. It just takes what it gets.
Which is better? Probably InnoDB, especially given recent improvements to the engine. I don't have a lot of experience with InnoDB, but I have considered converting to it and the concensus does seem to be that InnoDB needs tuning to get good performance out of it-- "needs" meaning "has to have", not just "highly recommended". However, that is second hand. InnoDB does not support FULLTEXT search like MyISAM does. However MyISAM's FULLTEXT is so god-awful that that point may be nothing but academic anyway. WordPress does not itself depend on any FULLTEXT features that I am aware of but some plugins might.