I haven't posted in a very long time. Been looking at posts again.
I use iBook USB rig or an AirPort Express optical rig for past few years. Simply awesome.
I use the "older" HeadRoom Micro DAC with Xin amp.
USB is pretty darn good, but since you have a MacBook Pro try to take advantage of optical that's what make the MBP more expensive that the standard MacBook. Also depending on the DAC you get with optical on Mac OS X you can use upsampling. I forgot how to do this. I believe it's through MIDI or something. I learned about it from TheSloth a long time ago, but I never tried it myself.
I find optical a bit cleaner. Also sometimes USB can have ground noise/hum depending on your rig and peripherals. Which is an advantage of a laptop Head-Fi rig: The entire rig can run on battery power, for super clean power.
Having said all that, if I was to do it again as of right now, I'd definitely consider the Pico first and foremost, then the Predator based on the reviews and posts I've read. Yes, it is only USB, but the reviews and impressions are outstanding. Iron Dreamer has tested many, many full-sized DACs, and he thinks the Pico is amazing. I definitely trust his opinions and experience.
Why the Pico? Mainly cause I like small stuff. A top-quality DAC/amp that super-duper small and for about $500. And it looks sexy too which goes well with MacBook Pro. For me looks count a little bit.
Combined with IEM's this is a spectacular super transportable rig that would be extremely cheap given the quality sound and small footfprint and great looks. You can also use the Pico with iPod or any other DAP.