The advantage of being an MT is that you CAN open your own business as indicated above. It's also a relatively short education...a LITTLE shorter than PTA, depending on your state requirements. The disadvantages include: it may be difficult to find employment, especially in an unstable economy. Very few insurance companies pay for massage outright. Therefore, most people pay for massage out of pocket, even if they are doing it for medical reasons. Another disadvantage is that its difficult to do massage as a full time career. It's a LOT of stress on the hands and arms. Very few people can do it as a full time career. Therefore, it's also hard to get benefits from an employer.
Being a PTA will be good employment in any economy, but will be subject to highs and lows when healthcare reform is in action. Your employment is more likely to be stable and getting benefits will usually not be a problem. The activities are much more varied as a PTA, therefore, it's easy to do as a full time job. The major disadvantage is that it SOUNDS like a stepping stone to becoming a PT, but it's really not (although neither is the MT a stepping stone to anything else). You also cannot typically be the business owner as a PTA (it will depend on your state requirements...in either case, you will ALWAYS have to work under a PT, but the level of supervision may vary).