They are both lossless. ALAC is simply compressed (like FLAC), whereas AIFF is not (like WAV, but with better metadata support). So basically it comes down to more storage (to hold the larger AIFF files) or more CPU (for converting the ALAC files). Hard drive prices are always dropping, and the CPU requirements are fairly low, so neither one may be a driver. Some have stated that they can hear a difference between the two, usually attributing it to the additional CPU overhead needed for compressed formats, but I'd imagine that you'd need an extremely resolving system to have a chance of detecting this. (In other words, if you haven't treated your room yet, for example, you have far more significant opportunities to improve your overall sound than changing a compressed to uncompressed lossless format.)
For what it's worth, I use AIFF both for my home system and the iPhone that I connect to my iFi iDSD driven headphone rig at work. (I have enough space on the phone for what I want to listen to at work.) Of course, I'm handling driver correction digitally before the signal gets to my analog line-level crossover, so perhaps I already have the CPU active enough that moving to ALAC wouldn't matter anyway. Regardless, I'm not going to consider changing it until I get my own room treatments finished.
Now that I think about, however, I'm likely to transition a 3rd generation AppleTV to network rendering duty in a second system (since I stupidly killed my modded Squeezebox Touch), and that unit might actually find itself CPU challenged with ALAS files. I'll have to play with it at some point...
