When it comes between the Diamondback Explorer and Type-6, there are a couple major things to consider. The Diamondback has a longer jump range and better combat ability, but the Type-6 is faster and able to load more equipment while still having appreciable cargo space.
I have a mostly A-rated Type-6 which makes a little over 27 Ly per jump with a full tank, and that is with a shield generator and SRV bay on top of the essential exploration equipment. It still has 64 tons of cargo space on top of all of that (80 tons if I unmount the SRV bay). I have never really had overheating issues, and it helps to shut off the shield generator when you exit the bubble. I could mount a larger fuel scoop (I usually use a 4A scoop), but it fuels reasonably fast nonetheless and I do often use the cargo space I get back from making that sacrifice (also note that the Diamondback has a maximum of class 4 but has twice the fuel capacity).
It sorta comes down to specialization over flexibility. If you just want to go far, fast, then the Diamondback is probably your best choice. If you like a bit of mission flexibility (including the ability to salvage things while out in the black), the Type-6 may serve you better. There is no wrong answer, though, just the one that works for you.
Another option, if you are okay with an even more stripped-down build, is the Hauler, which has the advantage of being cheap. Just a thought.