I have used both, and far prefer PBS. However...
1. The PBS parameters must be tightly controlled contractually. In order for this to happen your MEC, or their minions, must understand the system. Get this wrong, and life is not going to be so good for a while.
2. You MUST read the directions AND attend the training seminars AND particpate in whatever practice/training bids are run. If you attempt to do the typical guy thing and jump right in without reading the manual, be prepared to sacrifice several months of your life while you sort things out. I spent several unnecessary months on reserve...fortunately I was based in SLC and I like to ski

Me: Well, looks like reserve again in Feb. Wife: But I thought you were top 50%??? Me: I have to go catch my commute, have you seen my season pass?
3. Make sure you get automated trip dropping and swapping...that is an absolute godsend. Try to get contractual reserve staffing.
PBS will not affect pairing construction or rigs. It just builds lines out of available pairings.
If you have something like trip-touching for vacations, you will need to negotiate something to make up for that...obviously don't just give it away. Extra vacation days at a higher credit could make up for trip-touching. PBS should offer enough benefit to the company without concessions. Done correctly it should be a win-win.