If you wanted to learn a bit about MapThread then you could study this
A = {{1, 2, 3}, {5, 6, 7}, {5, 7, 9}, {3, 3, 8}};
B = {{2.3, 2.8, 2.5}, {3, 4, 10}, {4, 2, 3}, {9, 6.3, 3}};
f[u_,v_]:=If[Last[u] > Last[v],u,v];
MapThread[f,{A,B}]
or if you prefer this style
MapThread[If[Last[#1] > Last[#2],#1,#2]&,{A,B}]
