Here is my take on it (a little long...lol).
To preface my opinion, here is my situation. I currently have a 997 Humminbird, and have had it for three years. Before that I had the 987 Humminbird for about two years. The 997 has functioned fine, with no issues, but the 987 was a different story. I truly believe I had a lemon because of how many times I had to send it back. Most times though, Humminbird was good about getting the issues resolved and the unit back to me. I did have one issue that took a while to resolve, but I was eventually taken care of.
I also have a Lowrance 522, and have had it for about three years. About 3 years ago it took a dump on me and I contacted Lowrance. Since going to the HDS models, they offered me an opportunity to purchase an HDS 5 at a greatly reduced discount. I jumped at the deal, and have the HDS 5 on the front of my boat. It has been on there for a couple years now as well.
Without trying to dump on either brand, this is what my thoughts on these units are:
Fantastic customer service 95% of the time. Unfortunately I had to use it a good bit, but almost every time it was a very easy process.
I think the SI picture is tad better. In looking at the LSS vs. SI, I think the Lowrance image is just a tad too dark, and makes picking up contrasts on the bottom a tad harder.
The "Switchfire" program improved the 2d sonar a good deal. IMO the 2d sonar performance would have been something you would expect from a $200 unit, but not a $2000 one. Before the switchfire, I was very disappointed in the sonar, but after the switchfire upgrade, I don't really consider the 2d to be something that detracts from the unit.
Humminbird is more user friendly for someone not experienced/comfortable with operating menus. The menus are very quick and easy to navigate.
The mapping seems to work just fine, and I haven't had any issues with the 997. The only real dislike I have for Humminbird mapping is that when you are moving very slowly (like fishing offshore on the trolling motor) the directional arrow turns into a doughnut looking think, and you have to be moving about half a mile an hour to get the arrow pointing back to the direction you are going. Normally it's not a big deal, but at night when I am fishing offshore, it can be a little annoying.
With the Navionics Premium chip there is a little lag in the map refreshing as you move/zoom, but it is tolerable. I know with the Platinum chip and the lake master chips, it is miserable and don't count on it refreshing while you are running down the lake. When I make long runs, a lot of times I will take the chip out of my unit just so that it will keep up with where I am.
A better 2d sonar sonar picture. For the 2d the best comparison I can give is like regular TV to HDTV. Humminbird has some great images online, but watching both depthfinders during the same day, the HDS is a better 2d sonar picture. You can still see the same stuff, but one is just a bit prettier.
I feel the same for the DI picture as well. Lowrance achieves it's DI picture from a transducer that was designed to do such. Humminbird takes a portion of each SI beam and then splices the images together. While this works, I think having a transducer that is dedicated for such an image is why the Lowrance gives you a better DI image.
I haven't experienced any mapping or GPS issues with the HDS's while using them. However when you put a map chip in the unit, just like with the 'Birds, it seems like it makes the unit lag a little when you hit the buttons. It's not a major issue, but a little frustrating sometimes.
While the Lowrance is a little harder to learn how to operate, it gives you much more options on the way to setup the unit. You can pick exactly what data you want to be displayed, where you want it displayed and how big you want it displayed. This can also be different on each screen. With Humminbird you select what data you want to be displayed, and it is done so across all screens except for certain navigation and done in a way that it eats up a lot of screen space. For some reason you cannot remove your "heading" form one of the navigation fields on the Humminbird, and that is really irritating to me. To me, it seems like the Lowrance operates more like a computer with it's menu systems, so if you use computers a bunch, you can pick up on it pretty quickly.
The HDS is also going to give you a lot more usable screen than the 998. The way the 998 unit is setup to display information, there will be a big white box on the left side of the screen. With the 2d sonar showing, they are transparent, but in SI mode they are solid white and the available screen for the SI is much smaller.
Here is a screen shot I took with my 987 (same size unit, just older) that shows what I'm talking about.
So, purely on screen size, you will get a decent amount more usable screen with the HDS 8 in SI mode. Performance wise, they are both going to have their pluses. With that said, Humminbird is releasing an update that will remove the boxes from the SI screen and put the temperature and depth in the bottom left corner. This is what it will look like:
That significantly increases the usable screen space. The only problem is that you can't move the readouts around, so if you aren't happy with them, then you are SOL.
Customer service with Lowrance use to be TERRIBLE, but there was a huge internet revolt when they had their best customer service person quit (Linda Colt). Since then, Lowrance has made some pretty major changes (from what I have read) and the reports of their service seem to be getting much better. I have had to call a few times, and never had an issue getting through, so it has been good for myself.
The Lowrance seems to be a little better at canceling out interference on the sonar screen. I am not sure if this is from the broadband sounder or noise filters or some other reason, but the Humminbirds seem to have considerably more interference issues.
Take all that for what it is worth. Both brands have things I love about them and things I cannot stand. Either brand will do what you want it to do, and do it well. It's just a matter of which one you like better.
I will say, if I was looking to buy a new unit today, I would probably get a Gen. 2 Lowrance. They have a faster processor, and the lag you experience in the Gen. 1 units is greatly reduced. Plus, Lowrance is about to release the LSS-2 in July/August (according to the internet). Now this is all rumor (except the release of the LSS-2, that is happening) but supposedly the new transducer is going to be a bit longer and include another module. So, what that actually means, I can't tell you, but I'm sure it will translate into either new features or better performance. That also means that there will probably be a sale on LSS-1 kits as the new LSS-2 comes out, and you may be able to score one of those pretty cheap.
That's just my opinion though. No matter which direction you go, you will have a quality unit and will be more than happy with it. Also, if you ever want to check out either one, just let me know. We can hit the lake together one day and you can play with them both.