Their are a few more variables to consider first is any native life based on DNA as we know it. It appears to me as though mars was an inhabited planet that suffered a catastrophe like the island of Karatoga, with only microbes and possibly cockroaches surviving. The earth ships might be the first primitive rafts reintroducing higher life forms. Next is how we get there, if we go via Hoffman orbits (approx 2 yrs via chemical rockets) we will almost certianialy develop near 100% recycling via either plasma incinerator or super critical water oxidizer. In which case it will almost certainly be easer to setup up self contained biospheres on mars rather than teraform it.
If however we go via nuclear rockets or engineered antigravity then we need to worry about significant ecological impact.
Or if we find life as we don't know it.
then we have some really hard decisions.
