Joe Ricchio has written a comparative review of the pho at Thanh Thanh and Saigon for the Maine magazine blog.
I will say that when it comes to beef pho, I prefer the preparation at Thanh Thanh 2 (I will also admit to having eaten it six days in a row on multiple occasions). This is not to say that I do not greatly enjoy Saigon’s version. And if chicken pho is your thing, Saigon is the place to go. At the end of the day, finding a favorite pho is a personal matter and I suggest you try both of these spots for yourself. After all, they are right across the street from each other!