You can go either way here Michael.
To get a transducer that works with your 100SX you can use an XNT-9-20 transducer along with an AD-926 Transducer Adapter Cable.
Newer model sonar units have more features and a higher pixel count on the display which makes for a finer detailed sonar image. You could double the amount that you would put into the transducer and adapter cable and get a new color 5-inch color display sonar unit (HELIX 5 SONAR) or triple it and get a Sonar-GPS unit (HELIX 5 SONAR-GPS).