I think that each of these companys have their strengths and minimal weak points. Being in this industry for over 30 yrs you see most and hear all the bad and some of the good.
Each one of your units (899c si and HDS Gen 2) is what they call "End of Life" units that are being replaced by "New" technology. For example, the Humminbird 899 si (7" screen) was at $999 at the beginning of the year and you can soon get a Helix 9si (9" screen) for $999. Newer Technology and bigger screen for about the same money. The best deal I have seen on the 899 is at Bass Pro. They have the 899c si Nav combo for $829.99. This is the Humminbird 899 with a Navionics Plus card ($150 value). But for a few more $$ you can have the Helix 9si.
The HDS Gen 2's are also end of life units. Which means that the Gen 3's with a faster processor will and have come down in price to the level of where the Gen 2's were at in the beginning of the year. So look for the Gen 2's to be promoted for 4th Qtr of 2015 and first Qtr of 2016. Also the Gen 3's will support the new Structure Scan 3D which is pretty amasing.
I think that each manufacturer builds and services a quility product. It just comes down to personal preference and the all mighty $$$. Buy the best technology for what you can afford. Good luck my friend--this is not an easy task.
My suggestion would be to buy a unit with as many key features that you can afford and that fits your style of fishing. Screen size, pixel count, power, side imaging and down imaging are all key factors in buying a new unit. But, I would not buy just yet. Wait and see what key features and pricepoints are coming for 2016. I think you'll be glad you waited.