Personally...
...I think if there is Mars life it will be like extremophiles and thus existing in refugia, before and after human settlement. Mars is a very marginal world for life - on the surface - and improving the planet will open it up for Earth and Mars bugs. The deep biosphere can probably be kept pristine because access by terrestrial imports would be nigh impossible - wrong habitat entirely for the usual retinue that surround humans and hard to get at.
Adam