Hi Steve. I have had an 899 side imaging machine for a few years. I recent bought a helix 9 side imaging mega and a helix 7 side imaging mega. Mega imaging hands down shows more detail than the 899. I can tell you, after using the 9" screen, I dont want to even look at the 7. I mean on the 9, I frequently see the fish shadows on the bottom plain as day. As in i can see its tail fins and direction its facing, look that's a carp, look a catfish. I haven't with the 7 but I've used it once.
That being said, I would recommend saving a tad more and getting at least the same size screen of the 899 (helix7).
And YES, the mega imaging is worth it to me.