Dolphin M550 Robotic Pool Cleaner vs Dolphin M400 Robotic Pool Cleaner

Both are Dolphin robots that clean floor, walls and waterline and run on the MyDolphin Plus app, and both average 4.7 stars. The M550 adds a caddy, a 2-hour cycle and a 3-year warranty, so it suits owners who want a self-contained kit and shorter runs. The M400 has a third brush, a longer 2.5-hour cycle and a longer cable, and owners report 5 to 10+ years of use, so it suits larger pools and buyers who want scrubbing power over accessories. Neither is flawless: M550 owners call the app glitchy and the caddy flimsy, and M400 owners say it gets stuck on steps and ledges.

• Choose the Dolphin M550 if you want a caddy in the box, a shorter 2-hour cycle and a 3-year warranty for a pool up to 50 ft.
• Choose the Dolphin M400 if you want a third brush, a longer cable and a longer 2.5-hour cycle for a larger in-ground pool.