I am surprised to hear this of the 757 may be due to the size of the exit and flotation devices, as for the EMB145 there is only one door (stairs too) may prove to be an obstruction and would allow for more water to enter the cabin, the overwing sounds logical in it's case. The other factors are egress and the amount of PAX., in any egress you want to generally have everyone heading for the nearest exit to egress as quickly as possible.