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!