Some radios only run on batteries. If you leave those on in alert mode too long, the batteries will die. They should not be considered for long term alert monitoring. Some portables can run on both, but may not come with an AC supply. If you may ever want to take it with to work, on vacation, or just to another room, you may want to get a portable and just leave it plugged in on the nightstand most of the time.
Some portables are also weather resistant, for boating, camping, hiking etc.