If you can be certain that when hitting the water you will be fully conscious and able to inflate the jacket, a manual inflate would be fine.
The downside to a autoinflate is having it inflate at an inopportune time, say when it is sitting in your car and it decides to inflate while you're driving at 65 mph. Or worse, it inflates after a capsize trapping you in the cabin. (This happened on the Wingnuts disaster during the Chi-Mac race a few years ago).
My preference is for an automatic hydrostatic inflator. These are less sensitive to dampness and getting wet as they inflate based on water pressure. Mustang makes them among others. I have 2 with harnesses.
If you use jacklines, it would worthwhile to consider a back tow inflatable. There is only one manufacturer, TeamO. Here's a link:
The only lifejacket harness with patented Backtow Technology. The complete lifejacket harness for sailors. Shop direct with TeamO or find a retailer near you.
www.teamomarine.com
Even if you are not in the market for a PFD, take a look at the videos on the Team O website, Very informative.
Here's another video of the back tow in use:
On downside to the Team O PFD is they are not CG approved and so don't meet CG carriage requirements. Nonetheless, we bought 2.