I'm a 32 single black male and Denver and Austin are in the running for me so maybe I can offer you some advice
Born and raised in the South. Atlanta. Moved to San Antonio a few years back for my job in technology and now I am fortunate to have a high salary and opportunity to work remote. I've been to both cities but of course I'm closer to Austin.
Austin:
+Definitely a "young" city where people are coming from all over the country, especially California, to make it their home
+Thriving music scene
+Active nightlife
+Great career opportunities if you work in technology. Dell HQ here. Facebook, Google, IBM offices here and plenty of startups.
+Very active and health conscious people despite being in Texas "one of the fattest states"
+No state income tax in Texas which will offer you overall a low cost of living.
+Very tattoo friendly
-Overpopulation of the infrastructure which is causing a number of problems
-Increased traffic
-Housing is getting more expensive
-Traffic is getting progressively worse
-Hot summers. Nuff said
-Lacks diversity. Austin is incredibly white and I feel that it is geared more for the hipster scene and bro-grammers. Even the music scene isn't that diverse being mostly rock bands. I mean I'm saying this as a black male who have lived and seen more diverse cities. This has been posted to ad nauseum but you are seeing other ethnicities in Austin grow in population while blacks are leaving Austin for places like Houston or even outside of Austin. I think it has more to do with economics more than anything but it'd be nice to find more variety of food outside of Tex Mex and BBQ. Maybe I need to explore more.
Denver I've visited but I don't have enough experience to tell you too much about it. I'd say Boulder is tattoo friendly and Denver would be too. It is a very progressive city. I slightly like Denver more because I am partial to four seasons and it seems to offer a bit more diversity than Austin. Denver would be more expensive but you get more amenities like access to outdoor activities that Austin cannot offer.
I'm almost certain that there's a higher ratio of men compared to women in Denver so maybe that would work in your favor
But like I said I think there are better people here that can expound on Denver and I hope they chime in because I'm curious as well.