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MP3 is a compression method, while MP4 (m4a) is a container that could be one of several different types of streams. It's really comparing apples and oranges.
For the most part, you really can't tell the difference between an MP3 with a good encoding and a CD unless you've got really good ears and good equipment. Granted, the files encoded at the higher bitrates tend to be a bit on the large size compared to the average MP3.
I really haven't dealt much with the mp4 container, but it isn't an encoding method. For listening on a computer (with fairly large memory resources) I'd stick with keeping files in FLAC (a lossless encoding method, opposed to the lossy MP3 encoding method) but if you're looking for files to put on a portable player, you can usually get by with middling MP3 encoding.
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