It won't be an investment, it will be an expense since it looses value once you own it.
Both use the same frequencies and the transducer is the same size and shape.
Humminbird and third party mounts are interchangeable.
The SOLIX has a touch screen, Helix does not.
The SOLIX will share mapping with another SOLIX or ONIX, the Helix will not.
The SOLIX will share the Heading Sensor in a AS GPS HS antenna separate from the GPS, the Helix will not.
The SOLIX can have custom views created, the Helix cannot.
The Helix has adjustable view sizes for split screen view panels, the SOLIX does not.
The Helix will display full width 360 Front view, the SOLIX is about 2" less width: