well,
i guess potentially it is more secure because it is made by microsoft, from a privacy point of view. however flaws exist in the protocol anyway. so i guess it is personal preference, are you more comfortable with GAIM, or MSN? potentially third parties could submit your chats to another organisation etc... even though it breaks data protection. But secure, should be as secure as on Windows. but we cant really tell. unless you use a packet sniffer, maybe you could detect its features with that.
Microsoft® Messenger for Mac News - Gaim