Do you need both? No. Do you want both? Depends on your ultimate goals.
They are, after all, two completely different tools. Scanmaster is a Scan Tool. IOW, it scans data coming from the computer and displays them on a small "two item" display. You can move through the data points one at a time to look for anything out of the ordinary, limited of course to the data points available on the ALDL connector, and the fairly slow rate of date there (One frame every 1.5 seconds).
The Powerlogger is a data logging device that hooks directly to the computer's address and data bus, and monitors many more points of data. The Powerlogger also requires a laptop to monitor its operation and display it's data. It monitors/logs data at a MUCH higher data rate (18 frames per second). Powerlogger will run on Windows XP, Vista, 7.
Here's a sample display of one two of the available screens on Powerlogger.
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I recorded this data one day trying to find out how much boost I could run with my front mount intercooler on 93 octane only (NO ALKY).
Now, with all that said, many of us run both the Powerlogger (for logging data), and the Scanmaster for full time monitoring when one doesn't want to bother with having a laptop running. When using both, the Scanmaster gets a new "brain" chip, and it hooks directly to Powerlogger for its data input (faster scanmaster results). Any other questions, don't hesitate to ask.