Not because I have a older Balance myself but I would also advise no the the i3. the advantage it gives you is small, the headache it can potentially give you can be big. Now if you understand the potential for issues and are OK then go for it. As long as your comfortable with it, then its a good buy for you. Gear is expensive, you deserve to enjoy it. the system is admittedly very neat.
I would be concerned about longevity... especially salt water and those internal parts, standard BCD service is generally a non issue, but the i3, you will ought to get that serviced yearly, just my thought. Also note other similar systems have been around for awhile, dont see them much, says something about the standard inflator.
Im not sure if you can "downgrade" the dimension or make it dual, as the i3 push rod links to the dump valve per the literature so I dont think you can just attach a standard inflator hose to a dump valve as it will have other parts integrated with in the dump valve. Anyway just how I read it. Besides if going the i3 route then use it.
Most likely you wont have any issues, I would consider it myself but Im on the fence as to if I want the potential head ache of the i3 and service needed? Guess I would have to try one to see if I would be really "sold" on it as a "must have" verses option to get rid of my Octo and use a combo unit. The best advise is find a shop that offers satisfaction guarantee. Plan a dive and then purchase, if you dont like it you can exchange for the the Balance. Better yet find a shop that may have one to rent/try/demo.
In regards to the Atomic SS1... Aqualung/Seaquest has their own.... called AirSource it was an Editors pick too......
Off topic for those with wireless computers, the advantage is that they could have only 2 LP hoses. You will always have to have that Octo with the Dimension i3 setup. But honestly if in a emergency buddy situation a full Octo is easier to deal with. For deep diving I still feel more comfortable having a full setup.