Which is better for FCPX... .aiff or .mp3?

AIFF is better, since it is uncompressed.

You can import MP3 as long as you Transcode Media > Optimized Media. FCPX will convert the MP3 to WAV 16-bit 48kHz Linear PCM (the same as AIFF basically.) WAV is the "native" audio format for FCPX.

dastoelk wrote:

... and why?

mp3 could be made with a VBR - and that, on the long run, could cause synch-problems. Or, mp3 often use a different sample-rate than the mentioned 48kHz .. again, could cause synch-problems.

I find that I can drag a file from the Finder window into FCP X and it seems to play fine. When I check in the browser, i see it has been imported as an MP3 in the original media folder of my library. Does FCP X not automatically optimize it?

IT depends on what your import preferences are. The MP3s are original media and go in the original media folder. Optimized files are stored in another folder.

I have create optimized media selected....

TThen there should be an optimized version in the optimized folder.

There is! So, how come when I click reveal in finder it sends me to the original media version?

TThe application always takes you to original media. That's the way it's designed.

How do I switch the song to optimized media?

You don't have to do anything. If there is optimized media available, the application uses it.