It mostly comes down to achieving your best fit but also the handling of a dedicated Tri/TT bike can be better than a converted road bike depending on your position. A TT bike is designed to be ridden in a steep position with a lot of weight on the front end. If you ride a similarly steep position on most road frames with a fast forward style seat post you're really changing the weight distribution relative to how the bike was designed to be ridden. That can end being a really squirrely ride.

Dedicated TT bikes also tend to be a bit more aero but your S2 is no slouch in that department.

But if you can get the stack and reach you need on your current bike and like the way it handles in your best Tri position then you can certainly go that route. Realistically swapping back and forth from an aero bar to traditional road position is a PITA and not something folks want to mess with on a daily basis.

So one advantage of having dedicated road and TT bikes is you jump on one for daily training and your position is dialed for that type of riding and next time you get on it you don't have to spend time making sure it's set up correctly. This usually means folks actually train more in the TT position if they can do so on any day without a lot of setup time. That assumes you ride the road position frequently of course the converse is true, if you mostly ride in the tri position and your buddies want you to join a hilly group ride or weeknight training crit you may or may not want to (or be allowed to) roll the tri bike and then have to decide if the switch over is worth the hassle.

-Dave

Angles are more than a little different. I have my S2 w slammed stem, way forward seat (though standard seatpost) and flipped 5 degree stem/very low rise clip ons. Thought it was great til I got a proper fitting and into a nice comfortable position on a new P2. When I got fitted, I didnt think Id be in all that aggressive of a position, but all I know is thefront end is a lot lower, my seat way far forward, and my gluts and hammys get sore in a way my S2 never does. Id need a very negative rise stem to even begin to try and emulate this on the S2 (and everyone told me not to try). I love both bikes, and, if I had to only have one Id keep the S2. its a really great aero tt hybrid compromise (check out the second tour de france individual time trial). But def glad I have both.

I have toyed with both full tri bike and the nice road bike I had before doing triathlon. Like other posters have said, the fit and angles matter a lot. I found that I could get a pretty fast set up on my high end road bike. I have a 57 cm road bike on which I felt pretty spread out and comfortable in road position, but at first it was really hard to "convert" to tri set up. I rode one half IM at 18 mph, then worked on configuration a lot, dialed in the fit, and rode a 21 mph half IM in the new set up. To get aero I did the following:

- Lowered stem all the way. There were 30 mm of spacers that went on top.
- Flipped stem to flat (I had already done this).
- Put in a Profile Design Fast Forward Aluminum seat post. Best $85 I spent on this. Helped my hip angle tremendously -- very important since I have long 175 mm cranks on the bike. Seat angle went from 73 degrees to about 79, and felt so powerful (and was 3+ mph faster).
- Clip on aero bars.
- Aero helmet.
- Adamo saddle. There are mixed reviews of this out there, but it really worked well for me. Very comfortable.
- Aero wheels (not necessary, but I had access to some nice ones).

To put my bike in "tri mode" it was about $250-$300, and about an hour or so of tinkering.

Disadvantages:
- shifting while in aero is not possible from the clip ons, but this didn't matter as much as I thought it would. An actual tri frame would be more aero. Maybe I would be able to add another .5 mph, not sure.

Advantages:
- Cheaper than a new tri bike by a ton - about $2k for what I was considering. Cheaper than installing full aero bars and aero shifters by about $500.
- I like being able to climb on the hoods. Both halves were hilly and I like climbing. I also like descending in "traditional" road drops -- feels more stable.

I too like going on my drops for going 45+ mph. I just feel more stable. On aerobars i feel like my front wiggles/shakes too much at high speed. Is this due to the bike not being a full on tri bike? I am pretty far forward with half my weight in the front already.

After reading the other 2 posters, i am still leaning towards a new cockpit for tris. I dont ride in groups often so will probably change my setup once or twice a year.

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