"Best"? Not possible to answer that based on the data you've given. Type(s) of fish you'll be targeting and the depths that you'll be fishing. Greater depths, smaller targets, weaker targets (fish without air bladders, for instance) should decide the issue. Black box? I suspect that there is something unsaid here but the 585 works with either. The 600W transducer is priced at a fraction of the 1000W and offers the same options (tilt and temp). Generally, there is no finer or more reliable marine electronics than Furuno and I think that their equipment is worth every penny.
If in the budget and if you fish, I would really go with a chirp syst. Currently only Simrad, Garmin and raymarine offer it, but maybe Furuno might unveil something next month. look here The Marine Installer's Rant: My sonar is chirping, what up? for simple info on it. Transducer will be about 1200.00 alone.
I have the 585 w/1000 Watt Hang down Transducer. Excellent Combo. If you are putting it on a downeaster with a top speed of less that 25 knots. The hangdown unit is better that the flush mounted tilt unit. Furuno stated this to me during my selection. I top out @ 22 knots & can read bottom with this combo. Lastly the 1000 Watt hangdown unit only requires a 1 stem hole. The Flush mount 1000 watt is about 5 holeWow! That is a lot to fill at a later date should you make changes