Don't know if this is the best place to ask this (new here). This concerns the old "aluminum is harsh-riding" belief. I've found that not to be true on the current bike I'm riding. The ride is better than with the steel frame I had before (and components are identical). But I don't like the short top-tube on the bike and am looking now at a Caad5 frame, which is a better performance frame. My question is whether there is anything about the Caad5 that might bring out the harshness of aluminum that so many people talk about (but that I haven't experienced with my basic Nashbar Al frame). I'm 6'3, about 190 lbs. Thanks for whatever thoughts you have on this.

Race style bikes stiffness is about performance efficiency.

aluminum structures survive longer if they don't flex.

or like Airplanes, flex cycled below a replacement threshold.
they measure hours of service, then park them in the Desert..

Want stiff for sprints, and less stiff century rides?
maybe you can seek out a frame like Richey, or others.
main frame Aluminum, rear triangle in Carbon fiber composites,
as well as the fork..

In my experience (which is extensive but doesn't include carbon), tire size and pressure make more difference than frame material. I weigh about 20 pounds more than you do, and I do all my riding these days on 32mm or larger tires. I've done the same 25-mile RT commute for almost 30 years, 100 or more times a year, and there's no correlation between tire size and my times, whether I'm using 23mm or 41mm. Even if, say, 35s were a little slower, I'd ride them anyway. I'm only going to work...

I commuted for about 6 mos. on 38s. This wasn't necessary, because the pavement is good all the way, but I also used the bike for rough touring and didn't want to fool with swapping out tires. Then I stripped that bike down for painting and used my road bike for the commute--no panniers, just backpack--and I do find I arrive quicker by several minutes over the 11.5 miles. But at least part of this I'm sure is that when on my road bike I just want to pedal faster--it feels right. The experience has taught me that a commuter bike is of no certain configuration. It depends on the particular commute--whether the pave is good or crappy, whether most of it is bike path or in traffic, etc.

Then, on the comfort front, I'm really questioning whether steel has any advantage over aluminum. Doesn't this have a lot to do with head angle? My former Specialized Allez steel had 74 deg., while the Al frame that replaced it has 72.5 and is noticeably gentler over bumps. Steering on the Al is less responsive, of course, but it tracks by itself. Could it be that most Al bikes have had the steeper head angles, whereas steel bikes have come in a variety of angles and have therefore been reputed to be more comfortable?

The design of the frame matters. One can make steel frames stiff/harsh and AL frames less so.

We don't know what Nashbar frame you have (so we don't know what to expect regarding its harshness).

https://sheldonbrown.com/frame-materials.html