There has been considerable argument about this, but I think that everyone can reasonably concede that if a probe or some other unmanned source did, in fact, find proof of primitive life processes on Mars, then back contamination becomes a real issue.*

What would you rather see?

- Proof of primitive life on Mars, although it would really screw up the idea of colonizing for many years, maybe forever?

- A lifeless Mars, ready to be inhabited by humanity and maybe terraformed?

Note that the latter is a lot harder to know for sure. Finding one sure example of the former would be proof enough.

I would like to see the first thing happen.

If it means putting Mars on quarantine for full colonization or terraforming, fine. People might still be able to explore using properly designed suits and decontamination facilities. There's nothing wrong with sterilized real-time operated rovers and waldoes operated from Phobos Station, either.

The discovery of life on Mars would shift a major paradigm. If it was found to be sufficiently related to Earth life to prove panspermia theories, or whether it was something utterly new (either would be amazing), we would have two examples of living worlds instead of just one. The whole picture changes. The odds of life being almost everywhere that could even remotely support it goes up astronomically, no pun intended.

*IMO, "forward" contamination is a much bigger issue, but there it is.