I currently use DirectScan, have had it since the early 2000's. Still use it, and I tune my own car and burn my own chips. Getting 18 frames of data per second is light years better than the one frame per 1.5 seconds you get with a Scanmaster.
However, DirectScan can only be used on a laptop with Windows 98 or earlier, and the laptop must have a parallel port. Can't use it with Windows XP, Vista, 7, or 8. I have an old dedicated laptop for my DirectScan. Most people don't want to deal with a second ancient laptop like I'm doing.
If I were starting now, I would buy a PowerLogger without hesitation. Get the full 18 frames per second with lots of added capabilities that Direct Scan never had. I will probably be getting one soon.
For day to day monitoring, a Scan Master is nice, since you don't have to bring the laptop with you. If you can afford both the Scan Master and Power Logger, you would have it all.
HTH
Sent from my SCH-I535 using Turbo Buick