Personally, I'd take the ATL option. Been there and done it. Do, however, give yourself at least 2.5 - 3 hours transfer time (inbound from Europe) to be comfortable. You will clear immigration & customs at ATL and then immediately recheck your bag through to final destination. Then it's a simple short train ride to your terminal and gate. On arrival MIA you are a domestic arrival and soon on your way. Transfer time for the return need only be about 1.5 hours (again, to be comfortable). Your bag will be checked all the way through from MIA to MAN.