I am a lifelong sailor, sailing instructor, and liveaboard cruiser and have seen the good and bad of both PFDs and jacklines.
Inflatable PFDs with built in harnesses are the way to go. We are never underway without them on. If the seas are calm and the situation stable then we allow them to be removed while in the cockpit, but they go back on before any maneuvers or movement away from the cockpit. An example of why this is important occurred this winter as we approached an anchorage after a hearty sail upwind. The wind was about 15 knots and the seas were small but lumpy as we dropped the main at the edge of the anchorage. It is still not clear to me what exactly what happened, but I remember pulling up on the sail tie and the boat making a jerk so I lost my grip. My feet rose from the deck and I went flying. I expected to land on the side deck, but instead my shin hit the 30 inch high lifeline and I was cartwheeled into the water. My vest immediately inflated and I popped to the surface!
The point is that you never know when you might go in the water, or after hitting what object on the way. In my case I was uninjured, and since the sails were already down and the boat barely moving, I was able to swim to the stern and clamber back aboard. You need to be wearing your flotation anytime you are working, because it is when you are working that you are distracted and can find yourself unexpectedly in the drink.
I also really favor the built in harnesses. Often building waves and winds come up suddenly. If you have a built in harness all you have to do is grab a tether and clip in. With a separate harness you have to find the harness, put it on, and clip in, perhaps while conditions are rapidly worsening.
As for jacklines, the common practice of running them the length of the boat along the scuppers is a bad practice as if you are falling off the boat the jackline won't stop you until you are dragging alongside...not a happy place to be. You really want the jacklines to be as close to the center of the boat as possible, preferably combined with a tether length that causes one to fetch up short and drop INSIDE the lifelines. We've divided our jacklines into three "tracks". The first is on the floor of the cockpit and is easily reachable from the companionway. With a short tether you can just reach sit on the the coaming and lean on the lifelines. The second track run from the cockpit around the outside of the dodger to strong points near the centerline of the cabin house. The third track runs straight forward on either side of the mast to the foredeck. We use ORC tethers with two working ends and a quick release shackle at the chest. The object is to keep a falling sailor as close to the centerline of the boat and inboard of the lifelines if at all possible.
I sort of like the idea of a short tether on the mainsheet or reefing lines because they also keep the attachment point inboard and high, making it hard to get outside the lifelines. On a really small boat with relatively weak lines this wouldn't be strong enough, but on a 35-40 class boat I expect that the lines would have plenty of reserve strength for this use. Dedicated jackline are probably better, but this is a useful idea to keep in mind.