I've ran the Garmin 74SV and my dad has the 73DV Garmin on his offshore boat. I now run a Hummingbird Helix 7 w/ SI on my boat now. The Hummingbird has the better imagery especially when it came to SI and DI. The mapping software (Navionics in the HB vs Garmin) are pretty much a wash. I personally like having SI for longlining. I like trying new lakes and its nice to be able to see what you may have missed just 100' away and mark it and go back to it. However, when I'm fishing areas I know, 9 out of 10 I'm running my Map, 2D, and DI combo for longlining.
I will also note that when I ran my 74 and occasionally with my dad's 73 the transducer loses the bottom when you're on plane. Hummingbird has a smaller transducer and for my boat, never loses the bottom.