Long tubes will promote better exhaust scavenging, thus improving low end torque and all around breathing throughout the entire rev range. Your average street header with its longer primary tube length and 1 5/8 diameter make them ideal for street use.
With the right engine/power adder combination, shorties are said to improve the power band over LT's at the top end of the RPM range, but do next to nothing to improve low end torque in the process. This however does not apply to your average street build.
Generally, most people buy shorties because of their lower cost and ease of installation. As far as buying headers for performance application, stick with a long, or mid length header at the very least.