SA 333 Gr6 and SA 106 Gr B

The difference is an interesting topic for carrying out your own internet search.

SA 333-6 is a high-nickel alloy suitable for lower temperatures than SA 106-B. It is also in a different P-group for welding.

That would be Grade 3 (around 3%Ni?). I think Grade 6 is very similar to 106B, but impact tested.

ASME Section-ll Part A -SA-333/SA-333M is prescribed in Table 1 to the requirements as to chemical composition of A333 materials.As per the Table1 didn't mentioned percentage of nickel for A333 Gr6.
During the Manufacture time different method of heat treatment using for A333 and A106 as per the specification, it will be affected material toughness. Design minimum temperature of A333 Gr6 is below -50 ,hence A333 Gr6 material generally using for low temperature service. (ASME B31.3)Design minimum temperature of A106 GrB is below -29. (ASME B31.3)