The internet opened the doors of digital distribution. However, files in high quality (CD-Quality, and even better- the 24 bit files) formats are large, so digital distribution is done mostly with digital audio files like mp3 and AAC, which are much smaller. These formats are not lossless, and there are differences that can be heard between them and the premaster. As the network speed increases, we can see that the quality requirements of the internet stores and streaming services is going up; it's always a good idea to get the best possible resolution master.
When you are doing the mastering, you want to make sure that the mastering place uses a round-trip codec that allows the real time audition in the mp3 and especially AAC formats, to make sure these files will sound good. These codecs are AURoundTripAAC provided by Apple and Fraunhofer Pro-Codec from Sonnox. With them, you can hear how the files encoded off site (by iTunes, Tunecore, etc) will sound, and that is important. During the conversion, the encoded files will have audio glitches due to the inter-peak samples, so using these round-trip codecs detect the problems, and they can be corrected.
Asking for a mastered mp3 file makes sense if you need it for your website and promotion, but you may not do all the songs in an album, only what’s needed for promotion. However, with the future in mind, your best bet is to request a 24 bit/96 kHz master, or at least master with the same resolution like the mix (today actually everybody works at 24 bit, some at 44, some at 48, some at 96 and few at 192kHz sample rate). If the mix is at 24 bit/48 kHz, do not ask for up sampling, it doesn’t do any good! 24/96 is the preferred resolution for iTunes, and the reason is that the codecs used perform better with a file at higher resolution. Even tough the CD file has 16 bit/44kHz, and the resolution is way higher than an AAC or mp3 file, the codecs will create a better file if you provide the higher resolution master than the file from CD.
At last, for vinyl the files should be 24 bit, with higher rates than 44.1 KHz if available, but 16bit/44.1KHz can be used too; and for cassette 16 bit and minimum 44.1 KHz (yes, I know, who needs them anymore? … actually, they are coming back slowly).
