Well, OK, but you asked and the answer is......I dunno.
I still believe the 2D by Lowrance is superior and the GPS is superior in it's function and form. Having said that, Hbird has an update on the way that's supposed to address the slow refresh rate, but the functionality won't be affected - things like boat icon choices. Their down imaging is a software update and it's free though. Lowrance will be a black box, a new transducer and I'm sure some network cables and bits and pieces.
Lowrance will be more expensive, but scalable from 5, 7, 8, and 10". Hbird will only be in 5, 7 and 10".
If price is a concern, H'bird would be the way to go by about $100 with the 798 checking in right at $1,000. Lowrance would be $700 for the HDS5, plus a reported $400 for the black box making it $1,000. On the top end, I think Lowrance would be $2,300 for the 10base map, plus $400 for the black box for a total of $2,700. H'bird 1197 non nvb would be $2,589. Again' Hbird checks in on price.
Of course, the hidden costs would have to be added in - like networking components, mapping and software for your PC to manage it all, but I'd be they'll be pretty much in line and honestly, when you start topping in at north of $2K for a fish finder, a couple of hundred either way is no big deal.
If you handed me $5K and told me to get what I want, no strings, I'd really struggle right now. I'd probably sit on it until after the updates are out, digested and reported on here before I did anything. But if you forced me to, I'd probably go and see if the updated color scheme of 2D on the H'bird update was more to my liking. If it were, I'd go H'bird, if not, I'd stick with Lowrance.
As an added benefit of going with Lowrance, It'd tick Jeepster off. hehehehe.
Just reread the question, and really didn't address it in it's original form. My take on the Lowrance side imaging and down imaging feature is that it's really, really neat, but no better nor worse than H'bird's features. In essence, they are the same thing and appear to work equally well from what I've seen of them. Yes, I'd buy another one, but I wouldn't chunk good electronics to go chasing after the SI dream. However, if I had HD units, I'd definitely get the black box add on. And if I were buying new H'birds, I'd definitely gravitate towards the SI models.
Wannabe...