Both look similar but I've ready that Sa Pa is always packed with tourists. Any recommendations on one versus the other?
I suggest you go to Sa Pa.The northwest market town of Sapa is colorful and charming, providing the perfect oasis in the midst of a strenuous mountain trek or rice-paddy tour. The Gothic stone church at the center of town is a bull’s-eye in the center of shops and stalls, serving as a reminder of the town’s French missionary influence. Dine on Vietnamese or European-inspired cuisine downtown, and don’t miss the Saturday night "love market," one of the most potent and strictest single’s nights imaginable.
You could research via Sapatrekkinghomestay.com . Staff is friendly and helpful . Last week , I had been there.
Sa PảSa Pa is more popular than Mu Cang Chai District. Mu Cang Chai, Sa Pa have many local live here then they can help you explore beauty of Foggy Sapa better with full service. But Mu Cang Chai is something different challenge, if you looking for an adventure in North Mountain Vietnam thien Mu Cang Chai is an wonderful destination.
Sapa - The Towns in Clouds
Mu Cang Chai - Challenge yourself
If you don't find any tour that useful for you, check here: https://www.vietnamtourbooking.com/en/sapa/