There are certainly pro's/con's and the portable USB drive is best when going to work on a friend's PC.
However, for those that don't mind the extra room required for a "recovery workstation", these can be nice/quick options.
I don't go out on PC repair calls professional, I only do this to help friends and I've used them long ago in enterprise settings. For someone like me, the USB is definitely the easiest route. However... For those who do this professionally, it's definitely best to have both the USB + workstation that utilizes a raw HDD hardware/software solution like PC-3000.
Unfortunately PC-3000 is expensive, about $6000 USD last time I checked. There are a few alternatives that are less expensive such as MRT and Dolphin Data Lab. I've never used the new copycats and can't testify to how they work.