FLAC is better than MP3 simply because it is lossless. Some peoples state that they don't hear any difference between FLAC and high bitrate MP3 files. It's true for most people who don't use professional music equipment. With any pair of decent headphones you can easily tell the difference between a 128kbps and FLAC. Between 320kbps MP3's and FLAC's, the difference is more subtle, but still very much existent, especially when it comes to a good number of instruments playing at the same time, and a 320kbps MP3 encode suddenly sounds "crowded". This difference is incredibly noticeable with metal songs that rely heavily on cymbals for example, where they are always washed away by other instruments in crappy encodes.
My recommendations, listen to FLAC at the house and listen to mp3 on portables. Audio Converter Plus will help you to convert FLAC album to MP3. If you have decent audio equipment in your home, you can tell the difference between FLAC and 320bit MP3 or at least I can with my ears. MP3 seems to destroy the high end range of frequencies and makes instruments like cymbals sound washed out.