I have used both PRlog and PRweb and I find I use PRweb a lot more. I feel the distribution is better and I get a much better response. In regards to the power of the backlink I'm not sure as I haven't tested it, but I would probably recommend PRweb over PRlog for distribution. In saying that, I've had some real hassles with PRweb as they are really picky about the written PR, I have had a lot of problems with them telling me that my PR was not newsworthy, but when I question it they approve it. I've contacted them about it and basically it comes down to the editor who is reading it and approving it not based on their actual guidelines a lot of the time, I've never had any issues with PRlog and they seem to be a lot more lenient with guidelines on the written content and that kind of thing, so you may find a better click through on your PR's with PRlog for the simple reason that you can promote products a bit more within the PR, as with PRweb they seem to have some tight guidelines that makes it hard to do any kind of strong promotion. That's just my opinion with my experience, I do feel both services are great and I wouldn't hesitate in using either.