I have owned Big Jon downriggers for thirty years or so. Have three on the 22 cruiser. Four on the boat before that and two on the boat before that. They have always worked as specified. They have all been manuals. I have used a lot of electrics as well, but not my own. All experience is in fresh water not salt. Two of the riggers on my CD have four foot booms and one a five foot boom. If you were only going to run two riggers, then the shorter (about 2 foot) booms would be okay.
Big Jons are made about fifty miles from where I live so parts are readily available and you can get parts straight from the factory as well as most stores around here. I have never needed replacement parts, only additional parts such as adding an extension boom, longer screws, different bases, etc. I use their over-priced swivel bases, and the rod holders and the ball holders too.
You can raise the balls faster with manual than with electric but you can't do two at a time and it is harder to raise one while fighting a fish when you are single handing. But it can be done. The biggest advantage with the electrics to my mind is that Big Jons and some others have a nice controlled rate of descent. This lowers the lures, sometimes with several feet of flashers and etc, without tangling. The only complaint I have with my manual riggers is that the clutches takes some practice to lower the balls without going too fast or too slow. My fishing buddies will use their free hand or their belly to slow the spool so it won't have a sudden drop, I can do it with just the one hand on the reel but this could be improved upon --in my opinion. The biggest advantage electrics have over manuals is the descent not the ascent.
Have much experience with Cannons and they are good too. The riggers are as good or better than the Big Jons and if going electric Cannon might be my choice, you should check the auto stop feature on both. I like the rod holders better with Big Jon. Have used Walkers. Never used Penns nor Scotties
Good luck,
Mark