I've used both with the G9. In my experience, electrical sync cords are a 'PITA'. Expensive and prone to problems.
Using electrical will save battery capacity at the margin, but I'm not so sure about recycle time as the external strobes need to recharge as well - some external strobes are probably quicker than the internal, but not by much I wouldn't think. That was never an issue nor was battery life when I used to use optical with my G9. In either scenario (optical or electrical), I typically was able to do two dives with one set of batteries and only had to switch out for more than two dives.
I can also make my own optical cables for less than $20 each whereas electrical sync cords cost at least 5x that much ... and optical are more reliable in my experience.
Makes sense I suppose, but I've never liked the optical TTL options with my (admittedly older and lower end) S80. Waiting for the camera's on-board strobe to recycle takes a lot longer than the 1.5 seconds or so my Inon D2000 strobe needs... it's ready to go again far sooner than the camera is, but I still have to wait for the camera to recycle before I can take another shot.
Currently I run the camera and strobe in manual, because (a) I've never had good results from Inon's faked "S-TTL" and (b) the internal camera flash can be forced to 1/3 power which recycles far faster than a full dump. The internal camera flash takes MANY seconds (more than 3, less than 5... never timed it) to recycle from a full dump flash (such as on rare occasions I flip to auto/UW mode for a quick shot with no time to play with settings) ... an eternity of waiting while you're trying to get shots of a moving subject. Manual mode minimizes this, but but the internal flash still takes longer to recycle than the Inon strobe does, even with the Inon putting out full dumps.
Maybe these aren't valid concerns with the G9/10 over my S80, but they've certainly been sticking points for me with my rig. The G10 has certainly been high on my list of potential successors but issues like these, and the limited WA options, have been holding me back.
I generally do at least four 60+ min dives per day when I'm on a trip, and a battery tends to last approx 1.75 dives, so I end up changing batteries after every dive to make sure I don't end up with a dead camera partway through the second dive. Again, this is more an issue with my specific camera and may not apply to the G9/10 under discussion.