Well... that all depends. If your boat is set up correctly and you are happy with it's performance, don't bother with the hydraulic plate. However, if you are not satisfied with it's performance and/or want to be able to adjust your engine for changing conditions, props, etc... then the plate might be the way to go. If you go that way, do yourself a favor and buy a Bobs Machine Shop plate.
If you don't need the plate, and are contemplating the choice between the 8 and 7, my choice would be the 8. I like the larger screen, and I REALLY like the command soft keys (quick buttons) and flywheel on the 8 and 10. I find the 8 to be the perfect size too. The 7 will save you some money, and if you are happy with the performance and screen size of your 7, then you might want to consider going that way.
Hope that clears things up. :lol:
All the best,
Glenn