Pho 501 is very good. I like it better than Pho Boston, but it's much smaller. I got there around 11:20a on a Wednesday, and since I was a single, I sat at the "bar." 5 mins later, the wait was literally out the door.
Pho Boston is decent, but it's much bigger, so you won't have to wait to get seated.
Mrs Bigboy and I LOVE Pho 501. Order a spring roll app if they have it. Other than that 1 app, there's only a few items on the menu depending on when you go- Pho, Pho Ga, or Bun Bo Hue. They always have Pho and the Pho Ga or Bun Bo Hue are specials on the weekend that sell out quick.
There is often a wait but since the menu is so limited they can turn the tables very quickly as you are served just minutes after ordering.