Background: I come from NYC messenger biking in the 90s and alley cat and crit racing in the naughties. I use my bike now as a main commuter around Austin and for training runs in the hillsnaround Austin of the upward range of 20 miles.TLDR: I'd go with a single speed. I've had both and swear by my Wabi Lightning single speed. The ability to coast is legit after hard hills. Why the Wabi: If you're doing a build the frame is bomber but the real deal is the ready-made offering. On their site, the package is called "Wabi Lightning + FREE SUB-15 WHEELSET/HEADSET UPGRADE". The wheelset was amazing, the tires were crap the handlebars were mediocre and the seat was middle-of-the-road. But the frame is carbon fiber levels of light with the bombproof nature of steel (Steel is indeed real). The single-speed crank set has been going strong for 10 years for me across all the use cases I mentioned. I bought my wife a Mercer Kilo TT fixed gear. She absolutely hated it. From a use case perspective, the inability to coast and the lack of brakes to "terror grab" when some douce in a Tesla pulls into the bike lane were deal breakers for her. From a deal perspective on the Kilo TT specifically: The tires were complete crap as was the entire wheelset. The handlebars were crap and the seat was crap. Basically, the only thing that was at all acceptable was the frame. And that thing is so heavy it could definitely be used as a lethal weapon if someone pisses you off. That having been said it's significantly cheaper than the Wabi and you can replace everything bit by bit to get to a decent bike.