Line-out = 0dB and will not vary when the volume is adjusted.

Audio-out = -10dB to +10dB in most cases and therefore the volume level will vary when adjusted.

There isn't a problem with using Audio-Out but you have to make sure that you don't turn the volume up too high or you'll end up with bad distortion, clipping and possible equipment damage. Your safest bet is to get a volume meter and adjust the Audio-Out to 0dB, then it will be exactly the same as Line-Out ( as long as it's in stereo ). You can also do this with most recording software as you can set the input level or peak value of the sound before recording. Again it's better to keep the equipment volume down and bring the software gain up to avoid sound deterioration.