Separate
There are already over 80,000 posts in the RV-12 forum, more than any other RV. A search for info on a subject yields a lot of threads to wade through. Most of those regard issues that have been changed or updated in the iS airplane: the fuel tank, the sinking floats, the wiring harness, the back window, the MLG attachment, cabin heat, the voltage regulator, electrical capacity, fuel flow, fuel pressure, etc, etc. and about 35 Service Bulletins and more than 40 notifications. Many of the posts are re-plowing old ground.
Most of those will be of little help or interest to the iS builder.
Now the iS will introduce new issues unrelated to the earlier airplanes in areas of engine, maintenance, avionics, cowling, etc.
The 12 and 12iS look alike but inside are big differences in what Van?s furnishes in the kits compared to the kits of 10 years ago. That is not so true of say an RV-7, where kits are exclusive of engine and avionics choices, and I don?t think the situation is comparable to other RVs where different engine and avionics choices can be made by the builder.
Areas of commonality would be wing and tail cone construction but I don?t see that many posts regarding those. The fuselage, cockpit, avionics, engine are really quite different and that?s where the new posts are going to focus.
Yes the 12 and 12iS are similar, but for the reasons above I think the iS deserves a fresh start and so I would vote for a separate category in the forums.