If your goal is to knit tubular pieces with a steady rhythm, the circular loom makes it easy to knit in the round with no side seams and a stable stitch repeat. For scarves, panels and flat pieces to be joined later, the rectangular loom gives you control over back-and-forth rows and makes edges easier to align. Before you buy a loom, think about the projects you make most often and the space you have: the right knitting loom is the one that lets you work comfortably with a clear yarn path.
When you alternate between simple accessories and modular pieces, combining a circular loom for tubes and a rectangular loom for panels gives you versatility without adding technical complications. Choose formats that are quick to set up and put away, and focus on keeping the yarn path straight so the tension stays even. With that foundation, both circular and flat knitting become predictable, and you avoid interruptions to adjust the yarn path. The choice comes down to how you prefer to build and finish your projects.