Im not a bird diehard buy anymeans, I own an older lowerance that works great. I did buy a new 899 for the other boat as the features, performance and price were right what I was looking for. I also like its built in the US.
The 798 is a discountined model. Im not sure if it supports Lake Master, which is a one time purchase of $200, and allows you to map your own lakes. Lowerance offers their service which you pay monthly or yearly (not sure) and its crowd sourced data. Having the lake mapped for you is nice, though I prefer the one time purchase.
If your looking at the $700 price range, check out the 859. It doesn't do the SI, but honestly, IMO the SI is nice, but for the lakes around here, the 2d (old school) and down imaging works great. With SI you can tell what side the structure is on, the way the roadbed runs, the way the tree is leaning etc. DI covers 1.5 times the depth, giving you a wide view, the 2d will do coverage equal to depth.
The Raymarine Dragonflys are pretty cool as well. Check them out.
I looked, read and thought for about 2 months before I got the Bird 899, and for me its hands down the best fit. The only thing I'd change is a huge screen, but other than that, super happy. Just did a bunch of vertical jigging with it. Had also brought my Marcum LX9 to compare, and which cost quite a bit more than the bird and hands down I love the Bird better.
If you want to know anything else about the birds let me know.
Heres some good info.