The difference between Shark IW3511 and IW3615 Detect Pro cordless stick vacuum cleaners with auto-empty system are:
The IW3511 comes with a Brass color straight wand whereas the IW3615 comes with a black flexible wand which is helpful for reaching under furniture;
the Iw3615 comes with a crevice tool and a pet multi tool which is a convertible tool for cleaning tough pet messes, you can use it with or without stiff bristle brush, whereas the IW3615 comes with a crevice tool and a PetExtract Tool which “extracts pet hair and ground-in dirt from upholstery, carpets, car mats, car seats, and more, while also actively removing hair wrap.”

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Similarities between them:

• Powered by Lithium-Ion battery.
• Removable battery, runs up to 60 minutes.
• Self empty charging dock for charging and self emptying.
• 2L base holds up to 30 days’ dust and debris.
• HEPA filter and anti allergen complete seal system to trap and capture over 99.9% of fine dust and allergens down to 0.3 microns.
• DirtDetect sensors detect dirt and automatically increase suction power to pick up the dirt.
• EdgeDetect sensors detect edges and 2x the suction for better edge cleaning.
• FloorDetect sensors detect carpet and hard floors and automatically adjust the rotating speeds to fit different cleaning needs.
• LightDetect sensors detect low light spots and automatically illuminate dark areas for thorough cleaning.
• LED display shows dirt detection, battery status and mode type.
• 3 modes: Eco, Auto and Boost modes.
• Push button power control, NOT trigger type control.
• Self-cleaning brushroll reduce hair wrap issues.
• Odor Neutralizer Technology on the self empty base guards against bad odors.
• QuadClean Multi-Surface brushroll tackles different cleaning jobs on both carpets and hard floors.
• Lighter than previous full size Shark cordless stick vacuum.