Ditto what Knot and Flybye have said. You can still be tweaking fit when you do the tri - 15 miles isn't really *that* far (and you've been doing longer distances on your hybrid, no?).
I think a lot of it depends on whether you've decided (at least sort of) what you want, and whether there's a decent chance you'd find that used. I've bought 2 used bikes, and both have been good experiences. Of course, a grain of salt is in order
CA
Most days in life don't stand out, But life's about those days that will...
Normally late summer and fall is the time to get the best deals. There may be an issue this year as my LBS has told me they have sold a lot of bikes this year.
So I would go looking for a deal.
But...(hey, someone has to be the damper on the buy-party), buy it if the price is right and you get a good deal. Don't buy it for the speed. My gut says the bike shop is exaggerating. Depending upon the hybrid, if you're running narrower tires and smooth tread, your speed improvements won't be that dramatic. Yes, it will be faster; yes, you'll be more comfortable for the long haul; but in the end, it all depends upon the engine. I saw about 1-1.5mph switching from my hybrid.
That said, of course, by someone who is always planning her next bike purchaseIt is an addiction.
you can get slick tires absolutely. and it will improve the ride immensely.
you will NOT believe the improvement!
If you're worried about getting used to the bike in 3 weeks - I wouldn't worry about that. I did a metric century on a rented road bike because all I had was a mountain bike. I had no problems (except for the fact that the metric wa 2 days after I climbed Mount Princeton and my thighs were screaming - but that's another story). I wouldn't exactly recommend that, but it is doable.
As for the hybrid vs the road bike - you'll see a difference. Maybe not 20%, but you'll see a difference.
And when to buy? Whenever you feel comfortable. I agree that right now is the best time to get a good deal. Check 'em out and if you find one you can afford and absolutely love, go for it. If you don't, well, then Spring is when it was meant to be.
Christine
Life is not measured by the number of breaths we take, but by the moments that take our breath away.
Cycle! It's Good for the Wattle; it's good for the can!