@attran99 descriptions are spot on and both @JeetKuneBao recs are good starting points to explore these dishes…
As with most places in little Saigon you’ll find these two dishes on many menus just because they are really popular. However, as a general rule you’re better off trying to eat these dishes at specialist…
For bun rieu we try to stick with northern cusines specialist such as Quan bun ban mai. But often the lines get blurred and some places are just noodle specialists and offer all the popular soups…like mai phung which has a relative concise menu of soups featuring two types of Banh Canh cua, one with the annato infused slightly thick texture and one with a clear broth as well as bun rieu. So if you want to kill two birds…err crabs…
There is one Banh Canh specialist where the entire menu focuses on Banh Canh—30 different versions and that is Banh Canh que em & que anh. While most pple know Banh Canh as a crab and seafood soup, there’s actually lots of variety. We grew up eating not just the crab version, but chicken, and pork as well. So this spot has all all these varieties and then some.
We haven’t been to these locations in years so can’t comment on them now but it’s a good starting point for explorations of lesser known Vietnamese soups.