I just received a Pilot Custom 743 a couple of days ago, and I can already tell that I like it better than my 823. My reason for ordering it was I wanted to try out a Waverley (WA) nib, and had read several places that the nibs on the 743 and 823 could be swapped, both being #15 size Pilot 14kt nibs. My plan was to swap the nibs, put the fine nib I have on my 823 onto the 743, and then sell or trade the 743 with a fine nib.
Well, I swapped the nibs out, and the WA nib did not perform well on my 823 at all. It was dry, it skipped, it hard starting issues, etc. So, wanting mostly to try out the nib, I put it back onto the 743 and it worked beautifully. To my surprise, I found that not only do I like this particular nib quite a bit (it's mostly as side topic here, but I mention it just to give context for my feelings on the two pens), but I actually prefer the 743 overall as a pen.
Aesthetically, I like the 743 better, as I am not a huge fan of demonstrators, and I only bought the 823 for the "cool" filling system. Additionally, I do find the balance and weight of the 743 to be more to my liking. I like to post my pens, and when I post my 823, I find the balance to feel a little bit "off." Posting my 743 does not seem to change the balance for me, at least not that I can feel. The resin / plastic that the 743 is made of also feels a bit "nicer" to me than the 823's material. Finally, I have had trouble with the writing of my 823 even with the fine nib that came on it, ever since I acquired it a couple of months ago. It is a bit dry (even with the blind cap unscrewed fully), and the fine nib occasionally gives me skips and hard starts. The ink flow on the 743 is fantastic -- just on the wet side of medium, which is what I like, and it is consistent. The CON-70 converter holds a gracious plenty of ink for me, and having the option of using cartridges in a pinch is handy.
Sorry for any tangents there (I know you asked specifically about balance), but that's my personal experience with the Pilot Custom 823 and Custom 743, and I have come to the surprising conclusion that I like the 743 better overall. In fact, now I think I will be selling / trading the 823, rather than the other way around -- good stuff!
Best of luck with your decision, and have a nice day!