A shorter chainstay is stiffer than a long one, but all modern chainstays are a lot stiffer than ones of 20yrs ago. Longer chainstays are neccessary if you want to have clearance for wider tyres/fenders and heel clearance for panniers. Generally a longer bike will react more slowly, but that is also due to the more stable steering geometry needed on bikes designed for carrying heavy loads, or for endurance riding. It makes no real difference to your actual straight-line speed. Expedition tourers have loads of clearance and long chanstays. Racing bikes have them as short as possible. In between you get degrees of light-touring bikes which are excellent for both fast fun rides and everyday commuting. These are sometimes called Audax style bikes after the organisation which organises fast non-competative rides:
http://www.audax.org.au/
If you have the money, then Ti makes a superb bike, esp in the hands of experts like Lightspeed, but even the cruder Airbourne is a nice ride. Modern steel is also very good, and is usually better value than Ti, esp when it comes to custom builders.
What brands or manufacturers do you have in the shops.