I've spent a considerable amount of time in both. Ft Collins is probably going to be more left-leaning than what you're used to given that it's a college town. But Denver is really close politically as well. Culture-wise, there are a ton of transplants in Denver which means you'll probably be able to find something you're into. It's got a great scene when it comes to food, music, beer (probably the best in the country here), and outdoor living.
I've been a city-dweller for my entire adult life with a most of that being in SF. So from my urban-centric sensibilities, I think even Denver is a little too small for me. Denver has a lot going on but it's lacking in infrastructure. You're stuck driving everywhere. Plus the airport is like a 40 min drive outside the city (it's a little over an hour from Ft Collins to give you an idea) and it's very inconvenient. Now you might not care about these gripes at all, but they're things I have to deal with every time I visit.
The convenience of the mountains is fantastic - in both summer and winter. Whether you prefer hiking/biking or skiing/boarding. CO resorts are pretty pricey but I'm sure there are some cheaper ones locals are aware of. But most I've seen are more expensive than even Tahoe which is horrifying. But the mountain towns are very beautiful and most have a rustic old west charm to them. Great weekend getaway spots if you live in the area.
The music scene is probably the biggest driver for me. It's sick. Tons of quality local music coming out of there and everyone stops through. You have Red Rocks which is arguably the best concert venue in the world and a major destination stop for music tourists such as myself. I try to go at least once every other year. However, this music scene can attract some pretty crunchy people. I'm talking dirty wookie hippies. And this has only gotten worse since the legalization of marijuana. I'm sure I see this more often given the music I go there for (Phish, Umphrey's McGee) but Denver is a staple center in the US for this scene and I don't see that going anywhere. So you're in for what I assume would be a very non-TX crowd in that regard.
The beer. Oh man the beer. The concentration of breweries in Denver and Ft Collins is absurd. And it's some of the best around. Once again, I'm not terribly familiar with the TX scene, but it's very much a part of CO culture. A lot of the best restaurants even brew their own.
Almost everything here applies to both Denver and Ft Collins. You just get all the typical perks and cons for living in a smaller town in Ft Collins. Less people/traffic, more sense of community, less options/culture, dealing with the college influx and the desolation of summer. But Ft Collins is a pretty great place. I may even prefer it to Denver.