an mp3 has a tenth of the data on average that is in a wav file, ie compressed audio. Certain frequencies are removed to make for a smaller file while trying to minimise loss. Loss is inevitable with mp3, FLAC is a "lossless" format. I would recommend exporting to at least a 256kbps even better would be 320kbps when exporting to mp3. Wav is a lossless format which also has different bitrates going up to 192khz, maybe even beyond that. The balance is between quality/file size, personally I always export to wav and have a wav master from which all other formats are derived. That way, when recording vocals, I can give them a track right next to the master wav track and get a good mix, (the best will be to give the vocals their own track on the original mix session in pro tools/cubase/logic etc) but that is not always possible. Never mix in the mp3 domain, always wav, as a high a bitrate as your storage, cpu and interface will allow. If you ever collaborate with others it is always best to send wav files, otherwise you are compromising the quality before you even begin.