What is the difference between direct arithmetic computation and using MegaWizard blo

When I try to do addition, reduction, multiplication, previously I will use

plug ins in Quartus II like LPM_ADD_SUB. Then I will program use this addtion block to do addtion.The program will be:

module Test1121(out1,out2,a,b);

input [7:0] a,b;

output [8:0] out1;

output [15:0] out2;

add1(a,b,out1);

mul1(a,b,out2);

endmodule

However, It seems I can directly programing like this:

module Test1121(out1,out2,a,b);

input [7:0] a,b;

output [8:0] out1;

output [15:0] out2;

always@(a or b)

begin

out1=a+b;

out2=a*b;

end

endmodule

So I wonder what is the difference between them? Which one is better to be used?

Thanks