Isn't Titan better to colonize than Mars? It has a thick atmosphere which is made up of nitrogen like our planet's. It doesn't have a magnetic field, but it does get a type of protection from Saturn. Maybe the only downside I see is that it is too far away. While Mars has neither a thick atmosphere nor a magnetic field, it is bombarded with solar radiation every day.
We have no practical means to travel to either planet and set up even a base, let alone a colony. As such neither is better.
Also note that we have no reason to do it and we would need an extremely good reason because it would almost certainly involved the expenditure of vast resources to even set up a base, let alone a viable colony.
In a practical sense neither have any real human habitability options other that entirely sealed bases.
However current knowledge suggests that the geology on Titan would be really challenging. We'd be operating a base at significantly higher temperatures than the local norms and as such we'd always be subject to problems in a atmosphere with active weather. The only attraction on Titan is studying the locale, but we'd have done that in enormous depth before we could even consider a colony, so it's a mute point.
Mars is closer, less hostile and more familiar. We'd go there before Titan for the simple reasons it's easier, cheaper and more likely to succeed. But again, why ?
But we'd be unlikely to do either before we'd establish a viable self-sustaining Moon base of at least "colony" status. Again that's not even remotely possible and would not be possible without a unified global effort. Well this particular globe can't even agree the wording of one sentence in a document about it's own report on climate change. They can't even agree about one lousy word ! So no one is going to even start talking about the possibility of setting up a committee to discuss the likelihood of setting up another committee to start planing for a colonization effort research program.
In short, even if we solved the technical problems to make it worth deciding which place to colonize, we have no political chance in Hell of actually doing it. Humanity will die squabbling in a committee room. No colonists required until we replace humans with something that works. Skynet anyone ? :-)
I don't know if this question will make it past opinion close votes, but my favorite line about colonizing Mars came from Bill Nye in an interview. He said (and I'm paraphrasing) that we should think of Mars as like Antarctica. At any given time there's 700 or so scientists living on Antarctica doing various studies. But nobody wants to live there. Nobody wants to move to Antarctica and stay there. It's too barren.
He said he'd love to go to Mars if he gets the opportunity, but he'd want a round trip ticket.
I don't think either is a good colonization option. I don't disagree with a scientist who might want to go to Mars or Titan for the rest of their life, to dedicate their work to that - that's fine, and it's a choice. I'm not sure it's call it colonization.
A vacuum is one of the more difficult and expensive things to build technology for. Because Titan has an atmosphere, in many ways it's easier to set up a human base on. It's also a lot farther, so you have to either build faster spacecraft or deal with a several year journey and that's a huge disadvantage for Titan. A 6 month journey to Mars is hard enough to plan for. But once people get to titan, air tight is much easier to build than vacuum resistant.
Both have advantages and disadvantages. I don't think there's a true answer to which is better.
Mars is much better known, and that appeal could make it the next body in space that people land on, unless we land on an asteroid first or return to the Moon next. But Mars is being discussed and ideas are being explored. Titan, to my knowledge, isn't in that kind of stage of discussion, so Mars, may or may not be better but it's much more likely to be next in line.