There is no difference!
The two are completely equivalent - 'B' is just a symbolic name for the address 0xF0 in the SFR Area

Look at the generated opcodes:

0010 75F001           35             MOV B,#1    ; access of SFR 
0013 75F001           36             MOV 0xF0,#1 ; direct memory access (but does the same :)

They are identical!

That's why I says there are two distinct memory areas involved:
Direct addresses 00-7F access the bottom half of the RAM area;
Direct addresses 80-FF access the SFR area.