That is a good question. My wife and I live in Northern Liberties and specialize in all of the Riverfront areas. I personally believe that all of Northern Liberties (19123), Old Kensington (19122) and Fishtown (19125) will continue to do well and thrive. There is so much going on in these areas in terms of new development, arts and culture, dining and that will just continue. The quality of life is constantly improving.
From an investment standpoint, I would tend to believe that 19122 and 19125 will out perform Northern Liberties due to the fact that there is more growth potential and also that No Libs shot up in price too fast and furious. Thus, the other areas are attracting more interest. Further, many buyers are now priced out of No Libs so they want to be close to the area and are purchasing in these other areas fueling demand. Of course, the areas on the fringes or borders per se also will be a bit more speculative as well and thus a little more risky if by chance our market runs into a brick wall. With falling prices, people that would purchase would then come back to No Libs instead or Old City. I just do not think that there is much risk for our market hitting a wall, would tend to believe that it will be stable for another year or so and then start to pick back up again.
I know there is much debate over the borders and what is what so I think it comes down to personal preference and what you really like, what you can afford and what you are comfortable with. If you posted this same question on the Northern Liberties message board, I know you would get many many responses as well... as there is always debate about the borders and the surrounding areas.
I have heard that those units are quite nice as well. I wish you luck in your decision. For area residents, you will probably get 10 different opinions for the same question - is an interesting one : )
Good luck !