I have limited experience with Humminbird Side Imaging, but at least for DownScan I know Lowrance is much better since they use a dedicated transducer element for DownScan. Here is a ScreenCap from a Simrad unit using the old StructureScan LSS-1. LSS-2 is improved when it comes to deep water performance compared to LSS-1. The boat is MS Gudrun outside Flekkefjord. I also have a couple of pictures of Blücher which is at 90m.
Lowrance HDS is a far better deep water 2D sounder than any Humminbird. I have a TM260 which I've used with both HDS and Simrad NSS, both combinations have a very good performance.
I would recommend HDS-7 Touch instead of HDS-7 Gen2 + LSS-2. The Touch units have the StructureScan electronics of LSS-2 built-in. In case you prefer the seperate StructureScan balckbox, you should consider Simrad NSS instead of HDS-7. I have HDS-7 Touch, NSS-7 and LSS-1 StructureScan.(and a few other HDS units as well
I guess you are aware Humminbird and Lowrance HDS bought in US don't work very well in Europe. Both will lack Metric units, Scandinavian languages and the Lowrance will also show annoying pop-up messages.