Regular 16, Super16 and Ultra 16 all use the same film stock, the only difference is where the image is exposed within the frame of the film. Ultra 16 is a newer format that has some advantages. éclair ACLs are great candidates for both Super16 and Ultra 16 conversion.
A Super16 conversion widens the frame by shaving off one side of the gate so image extends into the part of the film that was originally intended for audio. Ultra 16 widens the frame by shaving off a little on both sides of the gate so the image extends into the perforation (perf.) area. A great article illustrating the difference can be found at here. Regular 16 has a frame ratio of 1:1.37 which is compatible with NTSC. Super16 has a frame ratio of 1:1.66 which is compatible with HDTV. Ultra 16 is a newer format and has a 1:1.85 ratio which is convenient for 35mm blowup. Ultra 16 is almost as wide as Super16, but also a little shorter which is why is has a wider aspect ratio. As far as resolution though, Super16 exposes the largest area of film.
Ultra 16 is a more affordable camera conversion because, unlike Super16, it does not require re-centering the lens mount. The total labor required and cost is considerably lower. Ultra 16 is backwards-compatible with Regular 16, meaning you can shoot either format after the conversion. Additionally Ultra 16 is compatible with all Regular 16 lenses, whereas not all Regular 16 lenses will cover the Super16 frame. If you have a collection of Regular 16 lenses, that can make a compelling case for Ultra 16. Lastly you can use double-perf. (two-sided) film, although that is probably not a concern for most.
If it’s so great, why isn’t everybody going to Ultra 16? Probably the strongest argument against Ultra 16 is that it requires special handling and not all labs support it. For digital transfer, the gate does need to accommodate the Ultra 16 frame, but more importantly the lab need to use processing equipment that does not have gears which could scratch the perf. area of the film stock.
If your preferred lab already supports Ultra 16 (or you don’t have a preferred lab) then it’s a great way to shoot higher resolution film for little investment. Ultra 16’s popularity is likely to grow as more cameras are converted which should lead to more labs supporting the format as well.