Actually DirectScan is more for the track. Data is only delivered to the ALDL port every 1.6 seconds. So, TurboLink can only show a frame of data every 1.6 seconds. Not a limit of TurboLink, mind you, but a limit of the data being streamed to the ALDL port by the ECM.
DirectScan plugs directly into the ECM of the car allowing it to pull data at up to 18 frames per second. That's 18 snapshots of your sensor readings every second.
Personally I believe DirectScan is more suited for the track because of it's high data rate. Once you start going fast enough down the track, TurboLink just doesn't show data quickly enough to get a good idea of how the car is running for the 12secs you are blasting down the track.
I have both, and I usually use TurboLink to diagnose problems, and DirectScan to record at the track.
TurboLink also has a nice GUI interface [not that I've used it cuz I'm too cheap a$$ed to upgrade, sorry Ken
] and provides alot of troubleshooting help that DirectScan does not. TUrboLink also work on a few others vehicles other than TurboRegals/TTA's.
What it comes down to is what you want. When I first got my car I bought TurboLink cuz to be honest I wouldn't have know what in the heck to do with DirectScan. After a few years and learning a WHOLE lot about these cars and the way they work I started using DirectScan.
I don't know if I cleared anything up there, but I hope that helps!