I was in that same spot last year. One thing everyone told me is to get the biggest screen you could afford. I'm not unhappy with my decision.
I then added a 7 to the front later because I got tired of turning around to see the 8. I'm still not unhappy.
Just think about it, on side scan you are trying to see 100' of info in 6 1/2" or 8" of screen. I say go for the big screen then you know you won't regret it. Those soft keys are a nice added bonus.