If you are most interested in sidescan mosaics, then you will need either a Humminbird SI model or a Lowrance with structurescan - although we are currently working on support for Garmin SideVu also.
Both the HB and Lowrance are good choices and you will get good results with either. As far as data quality is concerned, the larger (more expensive) units don't record better data than smaller ones, so the lower-end models are fine. All of our demo data, for example, was recording on a Humminbird 598 which is one of their cheapest sidescan models.