Regarding the aerobars on your road bike... the beauty of the TT bike is the double shifting while aero to get proper gear ratio.

You lose that advantage when you you're moving your arms (1 hand at a time) to shift.

(That's apart from the whole geometry differences).

(My thoughts regarding rank)

Primary is your engine.

Secondary is the chassis.

Third is the wheels

Therefore, I say full carbon TT with stock wheels will make you happier for comfort reasons.

 

Hmmm...I would have guessed that wheel upgrade would be the better $$ spent vs. frame material (assuming spending roughly the same amt.).  In that respect I wish bike mfgrs. would actually quote weights as/spec'd...so that at least in terms of that I could see if the carbon would be vastly lighter as-is or if lighter wheels on alum. would make a larger difference.

"better $$ spent"....

My rank may be related to the comfort rather than speed.

For speed purposes, the wheels may be better spent.

But, IIRC, you've had saddle issues with bikes before, and you might want to considering the comfort side of the ride and rank it higher than wheels.

I personally would lean towards the carbon framed bike for the comfort, especially over longer distances. The carbon bike will ride so much smoother. But if you are only going to do the occasional Du and not be too concerned with giving up some time to the advantages of a tri bike, I might look at getting some aerobars and/or a seatpost from Redshift Sports to convert your roadie into a semi tri bike for those events.

MTA: I'm also partial to BMC these days since taking delivery of my GF01 Di2

I went through this exercise in 2012 when I got my Speed Concept. I shopped and talked with folks for about 2 months before pulling the trigger.

The unanimous consensus I got was to to go carbon, not just for the ride but also for aero properties. Many people went as far as saying there is little advantage to an alumimum TT bike over just putting aero bars and fast wheels on a road bike. I can't vouch for that.

Some more things I learned:

- Mid-tier TT bikes intentionally come with heavy wheels that are mainly just good for training. This keeps the price down and allows riders to shift that part of the budget to whatever aero wheels they want or have.

- Weight (a pound or two) should be your last consideration in deciding between bikes.

- You can swap your nice wheels from your road bike to your TT bike. I do this with mine. They aren't super aero wheels but can still be a big upgrade.

If you have never ridden in the aero position I'd give that a go before dropping a couple grand on a bike. You can get a pair of Profile Century bars (rounded front and no stabbing in a pileup) for about $100. These are very conservative aero bars. They might buy you a little speed and let you know if you are comfortable in the position.

Comfort is key and be prepared to swap parts to get the position that allows you to stay aero as much as possible. If you can't stay in that position for long stretches there isn't much reason to have the bike.

Before I got by bike I did a multi hour professional bike fit. We narrowed down the frames & sizes that would work for me and then had to swap my stem and seat. The end result is amazing and I'm begrudgingly happy I spent the $$ on it. It's just a shame I'm not riding it these days but when I do it's like I would imagine it would be like pulling the Ferrari out of the garage when you spend the rest of the week commuting in a Honda. I'll get back to using that bike more someday and it will still be fast.

I feel like I don't ride anymore but actually do 4-5 times a week but it's just commuting on my little Brompton or tooling around on my CX bike but that's another story...

Yay, after 6 years of duathlons I'm finally getting a TT bike!!

The bad: not the bike I'd originally had my heart set on, as our shop lost its contract with BMC. That leaves KHS and Focus. The smallest Focus is at least 1 full size too big, on top of being an especially aggressive frame. Instead of smooth and light carbon the new ride may be heavier and sorta buzzy, as it's alum.

The good: the KHS is "dirt cheap," for the specs and should fit me well in an XS. It will leave some budget for wheel upgrade, too (or DH is looking to switch to a Focus road bike with sweet mid-depth carbon rims that he wouldn't use, so he'd hand those babies over to me). The BMC would not have. And I can someday upgrade the frame, assuming I continue to love duathlons for several more years...as much as I have for the past 6 years. Assuming the frame fits me well, I'd like to target my first Powerman du, next year.  Or maybe even this year, depending upon how the Summer race season goes and if I can stop catching every cold that comes along.