Tri bike or upgrade my roadie?

Without going too deep into it...learn about bike fits and their importance.  Lots of threads about them, so I won't go into detail besides encouraging you to strongly weigh a bike fit into your thought process.

Ok...that being said...on to your specific questions.

1.  Assuming you know which bikes fit you, looking used is a good way to get maximum value on a tight budget.

2.  Tri bikes put you in a more aerodamic position.  Thus making you faster when you're in the aero position (on the aero bars).  Other than that, a tri bike frame in itself is not really faster than a road bike.  Maybe a few seconds faster due to more aerodynamic frame shapes...but the frame doesn't make nearly as much a difference as people think.  It's certainly NOT lighter...they usually run about 2-3 pounds heavier than a road bike made of similar material.

3.  The theory about using different running muscles to improve your run is more of a myth.  There hasn't been any significant studies to actually prove it helps you run.  Most people who claim they suddenly run better now that they are using a tri bike are likely seeing the simple benefits of increased running and biking fitness now that they've been in the sport for a longer time.  You may also feel that you run better because you're simply spending less time biking.  If you're simply more aerodynamic, you'll go faster at the same effort level.  Which means you spend less time biking at that effort level...which means more energy left for the run.

4.  What bike you ride most of the time during training is up to you.  I prefer to ride my road bike whenever possible...but that's just me.  I also like to group ride.

5.  Climbing on a tri bike is a little more challenging.  First, the bike weighs more, and you don't have as many comfortable hand positions to choose from.  Your handlebar setup might also be a bit lower than you'd like to climb.  So while not ideal, it's not like it's impossible either to climb on a tri bike.

6.  I think you should know that the Slice is not meant to fit everyone.  In general, it's meant for people with long legs and a short torso.  Some tri bikes can be made to fit a wider range of body types...not so much the Slice.  If you're a short leg, long torso person...you'd probably shouldn't even consider the Slice.

Depends on what you want to do.  Tri bikes are made for non-draft racing, while road bikes are more versatile and comfortable for longer-distance rides. I love my Tri bike but really don't think I'd want to take on the Alps with it.  If you want to road ride 90% of the time then get a better roadie and use it for the occasional tri.  If what you really want to do is tri and need a roadie for the occasional ride with the buds, get the Slice.

Awesome advise, huh?

Thank you for all the information.  It's really leading me to upgrade my road bike instead of investing in a tri-bike right now.  I love climbing, and love group rides, which might not always want to include triathlon bikes.  This year, I'm doing a Sprint, an Olympic, and if training goes as planned, I might do a HIM in October, but not sure yet.  I think I can get away with a road bike for all three of those, and maybe then next year add a triathlon bike.  Might be a dumb question...but having a good road bike on a HIM should be fine right?  I can already ride 56 miles, but would incorporate bricks into my training to work on the running part after. 

Great advice actually.  I'm pretty much decided on upgrading my current road bike, and then see if I'm into tri's enough to buy a tri bike.  Maybe next year.  Thanks!

Great approach and what i would have recommended. Tri808 post is right on. The aero advantages of a tri  bike are somewhat exaggerated. Take a look at the Brits: they are kicking our mostly on road bikes with clip ons.

Uh ... draft legal?

I volunteered at the Boise 70.3 last year and saw as least two people riding mtn bikes! I was stationed at the turn and they were a fair bit ahead of the bike cut-off. So I'd say you'll be fine with your upgraded roadie. Hell with your budget you should be able to put together a really fine machine. Let the questions about which is the better upgrade begin!!!

PS: Don't ask me, I don't know those answers! :-)

I have the Specialized Tarmac and added the clip on aero bars and completed two 70.3's with them. Last year I bought the Cannondale Slice 5 and what a difference I am faster coming off the bike with what seems like less effort. The bike handles awesome smooth handle corners great seems to want to stay under you. I added the Speedplay Zeros, swapped the Fizik Saddle for the ISM Adamo Prologue and put on the Vittoria Diamente Pro tires. Tired of getting flats on the schwalbe's by just looking at them.

Good luck

According to Len brownlie, PHD Sports Aerodynamicist,  the savings are 17 seconds /40k for a TT frame.

http://www.active.com/cycling/Articles/Buying-Time-Which-Aero-Equipment-Offers-the-Most-Benefits.htm?page=2

I started on a Secteur Sport Compact.  Fine, but I hated the Sora (I know some people do just fine) and wanted a slightly more aggressive position.  I upgraded to a Canndondale CAAD10 105.  Love, love love it.  If you like the Cannondales, don't underestimate their ability to make a first class, race-worthy aluminum bike, while saving you money.

I can with an inverted stem. And i thought the whole point was aero profile reduction?