Ugh...

I guess I'll step in to clear up the ****-show and confusion.

First off, the OP isn't even asking about which frame would make a better road rider, so to give the answer to that is to ignore his actual question entirely. He's interested in riding a sw8 fixay on the street, not logging long, uninterrupted miles on a farm road.

Secondly, a frame can be both "stiff" and "compliant". How is this possible? The way a frame soaks up road vibrations is related to it's compliance; in that regards, a BK is a plenty compliant frame because of the tubeset used. A bike can also be stiff in the headtube and bottom bracket area, where stiffness is desirable by racers and anyone who intends to put a decent effort into their bike. Geometry and tubing affect both; the confusion in the conversation thus far has to do with confusion between these two things.

Tubesets aim to balance weight, stiffness and ride quality through diameter, butting and wall thickness; the BK's tubeset is, relatively speaking, a very oversized steel diameter ("standard"/classic being what you'd see on an old lugged steel bike), increasing the stiffness of the ride (in the sense that I just discussed). However, at the same time, the butting of the tubeset and thinnest of the walls at the most butted area add to the compliance of the ride, and make the BK soak up vibrations relatively nicer than similar steel bikes with thicker-walled/butted tubesets (Kilo TT comes to mind, as does my old Alien).

Now, the fun comes when you're trying to compare frames across different materials. The Leader will likely ride with both less compliance and more stiffness in the HT/BB area as a matter of the geometry and tubesets; especially with the fact that Leader likes to use aero downtubes, which really aren't known for a compliant ride. At the same time, the Leader will be lighter assuming all parts are kept the same, and will feel livelier on the initial acceleration due to the stiffness-weight ratio. (Compliance, of course, is not just down to frame material; tire choice, pressure and width make up the most immediate factor.)

Therefore, it comes down to values - do you want a bike that you can worry less about locking up, soaks up vibration from the streets a bit better but is also a little heavier and ultimately less stiff from a headtube/BB area perspective (but still plenty stiff for tree-trunk thighed trackies to race against in competition), or do you want a frame that may dent easier while locking up, ride harsher on rough streets but feel snappier under accelerations and not flex as much if you really sprint it hard?

For the record, I ride a BK and Concept, the BK mainly on the street and the Concept mainly on the track. Different tools for different uses, but I will say the geometry of the BK is among the best I've ever ridden (the BB drop is sky high and at least for me, that coupled with the front-end trail makes it a ton of fun to ride). If I could, I'd just buy another Concept to ride on the street but the extreme thinness of the tubeset's walls, while making the bike soak up vibration very nice for aluminum, also subjects it to dents while locking up.

Hope that was clear enough. The OP probably isn't gonna understand any of this for a couple more years.