That price seems quite high for the HDS-9 Live. MarineDeals has it for $3699 with that transducer.

We're running 2x HDS-7 Carbon on our 1550. Not a huge difference from the Live, other than some of the fancy Structurescan type functionality, which I find to be of very limited use, in the same fishery as you (mostly west coast, fishing out of Mana). I have used the *scan stuff about 3 times total. IMO it's a freshwater fishing feature, where you're running slowly up a lake or river edge on an electric trolling motor looking for submerged trees and such. I don't know that Garmin's offering is any better.

We have the Totalscan skimmer (functionally equivalent to the Active Imaging one for traditional sonar) and found it gives acceptable fishfinding performance out to about 80m, but recently added a TM185M and... wow. Ridiculous improvement. I can see a 200g jig in >100m of water.

Here's a screenshot that demonstrates the difference quite clearly - TM185M on left screen, Totalscan on the right. This is down at Verns, the fish being marked are Tarakihi.



If we did it all again, I would have bought the sounders with no transducer and immediately gone to the TM185M, or TM275LH if you want to do more deepwater stuff. We got a second bracket welded to the transom to accomodate the second transducer, and honestly unless you really really want the *scan stuff I would just run the one big transducer. The only tiny downside is the transducer ticks quite audibly.

Outside of all of that, we use CMap Genesis, where the chart is updated from the sonar, and it is a truly wonderful thing if you're fishing reefy structures like Hunters or Verns. I believe the Garmin is capable of similar functionality these days; if it isn't that would be a deal-breaker for me. The GPS performance is perfectly adequate; my understanding is that these units (regardless of brand) routinely outperform the stated GPS accuracy, especially when you have clear skies and you're getting a good signal from multiple satellites.

I have only had limited use of Garmin gear; from that experience I found the shallow-water sonar performance was adequate and the menu system was arguably better than Lowrance's, but I can't really comment beyond that.

The major downside of the Garmin is it's only 600W output, where the Lowrance does a true 1KW. If you want to get the most out of a big transducer, the Lowrance is the obvious option.

Whatever unit you go with, I strongly suggest enquiring about getting the unit without a transducer (or just flicking the transducer on Trademe if the economics work out) and going straight to a TM185M.