Do you live anywhere near one of the tandem speciality dealers who stocks tandems? Obviously, if there was any way that you could pay a visit to a tandem dealer who had a variety of tandems on hand that you could test ride you'd be better positioned to make a very informed decision about what tandem might suit your tastes a little bit better than another. Frankly, most of the recognizable tandem brands and most of the better smaller volume builders all build tandems that on a scale of 1-10 fall into the 8-10 range from their least to most expensive models... noting that an older 90's model from these same builders now occupy the 7-9 range only because of the lighter materials and components that are used today vs. what was available in the 90s.
Anyway, with regard to Co-Motion's tandems, the Speedster and Supremo use the same Reynolds air-hardened steel frame where the only difference is typically the brake mounts: Speedster uses cantilevers with bosses on the rear stay whereas the Supremo uses calipers with a drilled brake bridge. Forks are matched accordingly. The rest of the difference is specs and cost are related to the go-fast / lighter weight parts used on the Supremo to net it's somewhat lower overall weight.
FWIW, if you speak with the folks at Co-Motion who design their tandems they'll tell you that they designed their aluminum-framed tandems to have nearly the same ride characteristics as their steel frames, i.e., to have very similar, minimal deflection under load and quick handling. The primary difference between the steel and aluminum is weight: the aluminum frames are about 1.5lbs lighter and, well, the TIG welds aren't as small and tidy as the ones on their steel frames. The equipment specs for the Roadster are the same as the Speedster and the specs for the Robusta are the same as those used on the Supremo.
Co-Motion's tandems will feel quite different from your Ibis, at least at first. Your Ibis is more closely matched to a Santana Arriva with regard to how it handles and feels.