Does it matter (Bike Question)

The difference in speed is largely driven by a more aero position on the tri bike.  If you can't comfortably get a more aero position, then you will probably be faster on a road bike.  However, this would make you an exception as most people seem to be able to adapt to a better position on a tri bike.

As far as running faster, that's still up in the air.  There's one study (that I'm aware of) that showed there might be some benefit to running due to the steeper position of a tri bike.  But it's far from conclusive.

No, that's the study I'm referring to and I'm unaware of anything that's been done to study it more closely since.  Empfield notes some limitations of the study and there's been plenty of discussion about it in the past too.  It's an interesting hypothesis and at least another reason to consider riding steep.  Just not nearly conclusive enough for me on its own.

Your guys at the LBS is trying to sell you wheels.

bingo I think we have a winner.... are the guys telling you to get race wheels roadies or triathletes??

once I got a tri bike I could tell a difference when doing brick workouts immediately, It was less of the rubbery legs feeling for me..   it could be increased fitness gains, or a number of other things I'm sure but when I do a run after riding my tri bike it happens

Johnny, this is very interesting. People talk about a tri bike keeping your legs "fresher" for the run as if it were a foregone conclusion, but I've always wondered about that. My understanding of tri bike geometry is that it transfers more of the work to your quads and saves your hamstrings for the run. However, whenever I ran, even when I was thinner, it was my quads that seemed to take a beating from the run. This makes sense, at least to me, since the quads must absorb the eccentric load on the run, and eccentric loading causes muscle soreness. Either that or I have a really strange running form. Either way, I don't see the point of riding a bike in a quad dominant manner, then stressing the quads eccentrically on the run. On the other hands, all those triathletes can't be wrong, can they?

MikeTheBear, read the Empfield article linked above.  It discusses the study that leads to everyone's belief that this is true.  It also discusses why it might be so.  But it also notes some study shortcomings.  And if you have the time/desire, you should be able to find some lengthy discussions about it on ST forums.  Basically, at best it's a possibility but certainly not a well-tested theory.

Put me down in the unconvinced camp.  But there are other good reasons to ride a tri bike anyway...like it's faster for most people. 

Your bike shop guy is smoking crack. There are a gajillion posts and articles about what helps you go faster on the bike. Race wheels are well down the list, not far off trimming the length of your aerodrink straw. Seriously. Search the forums for the past articles, but I'll preview 'em.   In this order....

Great bike fit in a good position. Clips. Aero Helmet. Everything else.

You can also count me as a non-believer on the ranking of an aero helmet in your list.  And, yes, I know what the MIT guys did.

That said, it is a relatively cheap option and I ride with one.