There is one other advantage to three heads. If you make your own recordings you may want to monitor the recording. Only a three head deck allows you to compare in real time the recording with the original track. The construction of the head is also important. Try to look for ferrit or sendust alloys which are considered stronger and longer lasting.
Tapes have hiss - there is no getting around the fact.
Without getting bogged down in the technical aspects suffice it to say that the Dolby company found a way of getting around the problem. It generally gives you a noise reduction of 10dB., which though helpful is still far from optimal. About 10 years ago they came out with a new dolby called Dolby C which has a noise reduction factor of 20dB. Tapes recorded in Dolby C can be played on B and vice versa although there may be a little muting of the high range. Most decks today come with both options. The problem is in its encoding .